cmake: Add missed `Boost::headers` #34103

On Fedora 43:

$ gmake -C depends NO_LIBEVENT=1 NO_QT=1 NO_ZMQ=1 NO_WALLET=1 NO_USDT=1 NO_IPC=1
$ cmake -B build -DBoost_ROOT=depends/x86_64-pc-linux-gnu --preset dev-mode
<snip>
[363/1092] Building CXX object src/ipc/CMakeFiles/bitcoin_ipc.dir/capnp/mining.cpp.o
FAILED: [code=1] src/ipc/CMakeFiles/bitcoin_ipc.dir/capnp/mining.cpp.o 
/usr/bin/ccache /usr/lib64/ccache/c++ -DKJ_USE_FIBERS -I/home/hebasto/dev/bitcoin/build/src -I/home/hebasto/dev/bitcoin/src -I/home/hebasto/dev/bitcoin/build/src/ipc -I/home/hebasto/dev/bitcoin/src/ipc/libmultiprocess/include -I/home/hebasto/dev/bitcoin/build/src/ipc/libmultiprocess/include -I/home/hebasto/dev/bitcoin/src/univalue/include -O2 -g -std=c++20 -fPIC -pthread -fno-extended-identifiers -fdebug-prefix-map=/home/hebasto/dev/bitcoin/src=. -fmacro-prefix-map=/home/hebasto/dev/bitcoin/src=. -fstack-reuse=none -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=3 -Wstack-protector -fstack-protector-all -fcf-protection=full -fstack-clash-protection -Wall -Wextra -Wformat -Wformat-security -Wvla -Wredundant-decls -Wdate-time -Wduplicated-branches -Wduplicated-cond -Wlogical-op -Woverloaded-virtual -Wsuggest-override -Wimplicit-fallthrough -Wunreachable-code -Wbidi-chars=any -Wundef -Wleading-whitespace=spaces -Wtrailing-whitespace=any -Wno-unused-parameter -MD -MT src/ipc/CMakeFiles/bitcoin_ipc.dir/capnp/mining.cpp.o -MF src/ipc/CMakeFiles/bitcoin_ipc.dir/capnp/mining.cpp.o.d -o src/ipc/CMakeFiles/bitcoin_ipc.dir/capnp/mining.cpp.o -c /home/hebasto/dev/bitcoin/src/ipc/capnp/mining.cpp 
In file included from /home/hebasto/dev/bitcoin/src/node/miner.h:13,
                 from /home/hebasto/dev/bitcoin/src/ipc/capnp/mining-types.h:12,
                 from /home/hebasto/dev/bitcoin/src/ipc/capnp/mining.cpp:5:
/home/hebasto/dev/bitcoin/src/txmempool.h:28:10: fatal error: boost/multi_index/hashed_index.hpp: No such file or directory
   28 | #include <boost/multi_index/hashed_index.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
<snip>

This PR fixes the issue.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/34103.

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I can't recreate using the steps provided. At 5e7931af35732d1c0474f9e9d1adf270b850bd28:

# uname -a
Linux fedora-32gb-hel1-3 6.18.0-65.fc44.aarch64 <snip>aarch64 GNU/Linux
# gmake -C depends NO_LIBEVENT=1 NO_QT=1 NO_ZMQ=1 NO_WALLET=1 NO_USDT=1 NO_IPC=1
<snip>
copying packages: boost
to: /root/ci_scratch/depends/aarch64-unknown-linux-gnu
To build Bitcoin Core with these packages, pass '--toolchain /root/ci_scratch/depends/aarch64-unknown-linux-gnu/toolchain.cmake' to the first CMake invocation.
# cmake -B build -DBoost_ROOT=depends/aarch64-unknown-linux-gnu --preset dev-mode
<snip>
-- Generating done (0.3s)
-- Build files have been written to: /root/ci_scratch/build
# cmake --build build -j16
<snip>
[101%] Linking CXX executable ../../bin/test_bitcoin
[101%] Built target test_bitcoin

I can't recreate using the steps provided. At 5e7931a:

I forgot to mention that the boost-devel package was not installed. My apologies.

Why does the presence of a system package make a difference to compilation, if CMake has been explicitly configured to only take headers from Boost in depends?

Why does the presence of a system package make a difference to compilation, if CMake has been explicitly configured to only take headers from Boost in depends?

Because the /usr/include path, where system's boost headers reside, is included by the compiler implicitly.

Closing in favour of #34143.