manager: use correct cleanup function

Close the dir instead of attempt to free it.

Introduced in 874310b7b6

Found with coverity. Fixes: CID#996368

src/core/manager.c

@@ -896,7 +896,7 @@ static int manager_coldplug(Manager *m) {
 
 static void manager_build_unit_path_cache(Manager *m) {
         char **i;
-        _cleanup_free_ DIR *d = NULL;
+        _cleanup_closedir_ DIR *d = NULL;
         int r;
 
         assert(m);