core: keep track of the mtime of the transient unit file we wrote

Otherwise "systemctl status" will immediately report that our unit file is out
of date.

src/core/unit.c

@@ -1243,6 +1243,8 @@ int unit_load(Unit *u) {
 
                 fclose(u->transient_file);
                 u->transient_file = NULL;
+
+                u->dropin_mtime = now(CLOCK_REALTIME);
         }
 
         if (UNIT_VTABLE(u)->load) {