(griffin.c) Fix build when freetype2 support is enabled

This commit fixes build issues for frontends that wish to use freetype2 or that do not want to use the built-in bitmap font:

* Having `HAVE_FREETYPE` without `DONT_HAVE_BITMAPFONT` makes griffin.c include both freetype.c and bitmapfont.c resulting in a redefinition of `struct font_renderer` (freetype.c:25 and bitmapfont.c:24).

* Having both `HAVE_FREETYPE` and `DONT_HAVE_BITMAPFONT` resulted in linkage problems because in this case griffin.c does not include fonts.c and thus `font_renderer_create_default()`  implementation is missing. If fonts.c is included, the linker complains about undefined reference to `bitmap_font_renderer` in fonts.c:27.