Avoid `-Wundef` compiler warnings #149

Fixes https://github.com/bitcoin/bitcoin/pull/30975#issuecomment-2633222863:

On my own macOS 15.3 with Xcode 16.2 machine building fails:

git clean -dfx
cmake -B build -DENABLE_IPC=ON
cmake --build build
...
/Users/sjors/dev/bitcoin/src/ipc/libmultiprocess/src/mp/util.cpp:22:5: error: '__linux__' is not defined, evaluates to 0 [-Werror,-Wundef]
   22 | #if __linux__
      |     ^
/Users/sjors/dev/bitcoin/src/ipc/libmultiprocess/src/mp/util.cpp:62:5: error: '__linux__' is not defined, evaluates to 0 [-Werror,-Wundef]
   62 | #if __linux__
      |     ^
2 errors generated.

That does the trick for me locally, thanks! Will try in bitcoin/bitcoin#30975

Code review ACK d450fbf63ef4308dbeb164d9999e0c09f9ec5c91