RAW loader: Don't unload loader

Running with valgrind or asan seems to show that unloading/reloading the
loader causes memory leaks. Not dlcose()'ing the module avoids this.

src/modules/loaders/loader_raw.c

@@ -128,4 +128,4 @@ _load(ImlibImage * im, int load_data)
    return rc;
 }
 
-IMLIB_LOADER(_formats, _load, NULL);
+IMLIB_LOADER_KEEP(_formats, _load, NULL);