[backport] Fix the build with ZMQ disabled
ZMQ is conditionally linked to the server library, so executables linking against server will get ZMQ as needed.
The wallet library unconditionally links against zmq since D5840, which can cause the build to fail if ZMQ is disabled and the linker cannot solve the -lzmq flag.
Test Plan:
cmake -GNinja .. -DBUILD_BITCOIN_ZMQ=OFF
ninja all check check-functional
Differential Revision: https://reviews.bitcoinabc.org/D5872