depends: Fix CMake-generated `libzmq.pc` file
This is a backport of: https://github.com/zeromq/libzmq/pull/4706.
Similar to https://github.com/bitcoin/bitcoin/pull/30488.
Addresses https://github.com/bitcoin/bitcoin/pull/29723#issuecomment-2239864170:
Looking at the mingw .pc generated by this PR:
Libs: -L${libdir} -lzmq Libs.private: Requires.private:It looks like we'll need to take zeromq/libzmq#4706 as well for CMake. That can be done as a follow-up though, as it's not yet merged upstream.