CMake: fix: link against pathplan when building GD plugin

CMake: fix: link against pathplan when building GD plugin

This replicates something from the Autotools build system that was missing in
the CMake build system. The CMake build was not failing because the pathplan
dependency is only required when VRML support is enabled, which an upcoming
commit will add.

In contrast to the Autotools build system, this adds pathplan for all platforms,
not only Windows. Empirically it seems necessary on at least macOS and Windows
and it does no harm on Linux.

Gitlab: related to #1786