s/PROJ_INCLUDE_DIR/PROJ_INCLUDE_DIRS

Recent versions of PROJ (like the 9.2.0 I'm currently using) isn't exposing `PROJ_INCLUDE_DIR` but `PROJ_INCLUDE_DIRS` in the CMake configuration exported.

I'm not sure how multiple versions of PROJ should be handled if retro-compatibility with PROJ 6 is still desired and supported.