user-runtime-dir: fix selinux regression

Fix #9993.  When this code was split out to user-runtime-dir, it forgot to
include the call to mac_selinux_init().  So mkdir_label() stopped working.

Fixes: a9f0f5e501 ("logind: split %t directory creation to a helper
unit")

src/login/user-runtime-dir.c

@@ -10,6 +10,7 @@
 #include "mount-util.h"
 #include "path-util.h"
 #include "rm-rf.h"
+#include "selinux-util.h"
 #include "smack-util.h"
 #include "stdio-util.h"
 #include "string-util.h"
@@ -171,6 +172,12 @@ int main(int argc, char *argv[]) {
                 return EXIT_FAILURE;
         }
 
+        r = mac_selinux_init();
+        if (r < 0) {
+                log_error_errno(r, "Could not initialize labelling: %m\n");
+                return EXIT_FAILURE;
+        }
+
         umask(0022);
 
         if (streq(argv[1], "start"))