Remove unneeded include dirs in zeromq CMakeLists.txt

[timwoj]

I did the same thing in the Redis storage backend as part of https://github.com/zeek/zeek/pull/4567, but left this for a separate PR. This changes the ZeroMQ CMakeLists.txt to not directly add CMAKE_CURRENT_SOURCE_DIR and CMAKE_CURRENT_BINARY_DIR to the target for these reasons:

• CMAKE_CURRENT_BINARY_DIR is already added by default in cmake/ZeekPluginStatic.cmake.
• Files included that are in the zeek repo should be included with their full path (see the change to cluster_backend_zeromq.bif), so we don't need to include CMAKE_CURRENT_SOURCE_DIR either.