selinux: create /run/user/${USERID}/systemd with default context

src/core/main.c

@@ -2087,7 +2087,7 @@ static int initialize_runtime(
                         return log_emergency_errno(r, "Failed to determine $XDG_RUNTIME_DIR path: %m");
                 }
 
-                (void) mkdir_p(p, 0755);
+                (void) mkdir_p_label(p, 0755);
                 (void) make_inaccessible_nodes(p, UID_INVALID, GID_INVALID);
         }