Cannot build sysutils/synergy

Hello.

I'm building my ports in poudriere (13.1/amd64 host and target) after I 
upgraded to 2022Q4 and I have troubles with Synergy.

> CMake Error at /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
>   Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
> Call Stack (most recent call first):
>   /usr/local/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:594 (_FPHSA_FAILURE_MESSAGE)
>   /usr/local/share/cmake/Modules/FindPkgConfig.cmake:99 (find_package_handle_standard_args)
>   src/lib/synergy/CMakeLists.txt:51 (find_package)
> 
> 
> -- Configuring incomplete, errors occurred!

I think this is because I turned off AVAHI_LIBDNS option and in Makefile 
we have:
AVAHI_LIBDNS_USES=              pkgconfig

However pkgconfig is needed in any case, so it should go under plain USES.

With this change, it configures and builds (although I haven't had the 
chance to test it yet).

  bye & Thanks
	av.