glib/giowin32.c: Add G_GNUC_FALLTHROUGH annotation

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;

glib/giowin32.c

@@ -1661,7 +1661,7 @@ g_io_channel_new_file (const gchar  *filename,
             mode_num |= MODE_PLUS;
             break;
           }
-        /* Fall through */
+        G_GNUC_FALLTHROUGH;
       default:
         g_warning ("Invalid GIOFileMode %s.", mode);
         return NULL;