CMakeLists.txt: fix linking for macOS

[barracuda156]

Fixes linking failure:

[100%] Linking CXX shared library libwsnet.dylib
/opt/local/bin/cmake -E cmake_link_script CMakeFiles/wsnet.dir/link.txt --verbose=ON
Undefined symbols for architecture arm64:
  "_SCDynamicStoreCopyValue", referenced from:
      wsnet::getDnsConfig_mac() in getdnsconfig_mac.cpp.o
  "_SCDynamicStoreCreate", referenced from:
      wsnet::getDnsConfig_mac() in getdnsconfig_mac.cpp.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)