build: Add missed gcov files to 'make clean' #19718

On master (b4d0366b47dd9b8fe29cc9a100dcdf6ca1d3cabf):

$ ./autogen.sh
$ ./configure --enable-lcov
$ make && make cov
$ make clean
$ find . -name '*.gcno'
./src/rpc/libbitcoin_server_a-blockchain.gcno
./src/rpc/libbitcoin_common_a-util.gcno
./src/rpc/libbitcoin_common_a-rawtransaction_util.gcno
./src/rpc/libbitcoin_server_a-rawtransaction.gcno
./src/rpc/libbitcoin_util_a-request.gcno
./src/rpc/libbitcoin_server_a-net.gcno
./src/rpc/libbitcoin_server_a-server.gcno
./src/rpc/libbitcoin_server_a-mining.gcno
./src/rpc/libbitcoin_server_a-misc.gcno
./src/rpc/libbitcoin_cli_a-client.gcno
./src/node/libbitcoin_server_a-coinstats.gcno
./src/node/libbitcoin_server_a-transaction.gcno
./src/node/libbitcoin_server_a-context.gcno
./src/node/libbitcoin_server_a-psbt.gcno
./src/node/libbitcoin_server_a-ui_interface.gcno
./src/node/libbitcoin_server_a-coin.gcno
./src/test/util/libtest_util_a-setup_common.gcno
./src/test/util/libtest_util_a-net.gcno
./src/test/util/libtest_util_a-blockfilter.gcno
./src/test/util/libtest_util_a-mining.gcno
./src/test/util/libtest_util_a-transaction_utils.gcno
./src/test/util/libtest_util_a-wallet.gcno
./src/test/util/libtest_util_a-str.gcno
./src/test/util/libtest_util_a-logging.gcno
./src/index/libbitcoin_server_a-txindex.gcno
./src/index/libbitcoin_server_a-base.gcno
./src/index/libbitcoin_server_a-blockfilterindex.gcno
./src/util/libbitcoin_util_a-error.gcno
./src/util/libbitcoin_util_a-rbf.gcno
./src/util/libbitcoin_util_a-message.gcno
./src/util/libbitcoin_util_a-time.gcno
./src/util/libbitcoin_util_a-moneystr.gcno
./src/util/libbitcoin_util_a-url.gcno
./src/util/libbitcoin_consensus_a-strencodings.gcno
./src/util/libbitcoin_util_a-settings.gcno
./src/util/libbitcoin_util_a-system.gcno
./src/util/libbitcoin_util_a-threadnames.gcno
./src/util/libbitcoin_util_a-fees.gcno
./src/util/libbitcoin_util_a-asmap.gcno
./src/util/libbitcoin_util_a-strencodings.gcno
./src/util/libbitcoin_util_a-string.gcno
./src/util/libbitcoin_util_a-bytevectorhash.gcno
./src/util/libbitcoin_util_a-bip32.gcno
./src/util/libbitcoin_util_a-spanparsing.gcno
./src/util/libbitcoinconsensus_la-strencodings.gcno
./src/interfaces/libbitcoin_wallet_a-wallet.gcno
./src/interfaces/libbitcoin_util_a-handler.gcno
./src/interfaces/libbitcoin_server_a-chain.gcno
./src/interfaces/libbitcoin_server_a-node.gcno
./src/crc32c/src/libcrc32c_a-crc32c_portable.gcno
./src/crc32c/src/libcrc32c_a-crc32c.gcno
./src/crc32c/src/libcrc32c_sse42_a-crc32c_sse42.gcno

This PR fixes this issue.

ACK 6cb8771173d835bdeb89c76b1d53191d3c896968 -- patch looks correct

Is there an alternative to doing a mix of enumerate-all-folders, but then wildcard on the file name? Maybe a recursive wildcard or some kind of rule to delete only cov files that could have been generated while compiling the target?

Similar to $(BITCOIN_TESTS:=.log). Not sure if this is possible, though