lib/gvpr: use GVDLL instead of _WIN32 to control gprDisc init in compile.c

lib/gvpr: use GVDLL instead of _WIN32 to control gprDisc init in compile.c

Without this change, an upcoming commit that changes cgraph.h to use
GVDLL instead of _WIN32 to control storage-class attributes would
cause this error:

compile.c:85:29: error: initializer element is not constant
   85 | static Agdisc_t gprDisc = { &AgMemDisc, &AgIdDisc, &gprIoDisc };
      |                             ^
compile.c:85:29: note: (near initialization for ‘gprDisc.mem’)

Towards https://gitlab.com/graphviz/graphviz/-/issues/2173.