Fixes the following warning on CLang:

  ../glib/glib/giowin32.c:1665:7: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
        default:
        ^
  ../glib/glib/giowin32.c:1665:7: note: insert '__attribute__((fallthrough));' to silence this warning
        default:
        ^
        __attribute__((fallthrough));
  ../glib/glib/giowin32.c:1665:7: note: insert 'break;' to avoid fall-through
        default:
        ^
        break;