Remove HAVE_SGEN_GC dependency in mono/mini/main.c to fix build

A regression in mono/mini/main.c added a dependency on the HAVE_SGEN_GC define, which is never defined (and would cause an error in mini.h if it were). This was used previously to switch between including buildver.h or buildver-sgen.h depending on which gc was selected. After this change, buildver.h is always included, regardless of the gc being built, removing the dependency on the HAVE_SGEN_GC define. The make target for buildver-sgen.h now forces a rebuild of buildver.h, to avoid using a stale copy if both boehm and sgen targets are built, but the buildver-sgen.h file is never used.

Contributed under MIT.