deploy: Bump the mtime on ostree/deploy after deployments finish

This allows other processes (e.g. rpm-ostreed) to monitor for external
changes (e.g. if someone does `ostree admin undeploy`) in a relatively
sane fashion.

Specifically, I'm trying to fix:
https://github.com/projectatomic/rpm-ostree/issues/220

src/libostree/ostree-sysroot-deploy.c

@@ -1855,6 +1855,13 @@ ostree_sysroot_write_deployments (OstreeSysroot     *self,
                                 requires_new_bootversion ? "yes" : "no",
                                 new_deployments->len - self->deployments->len);
 
+  /* Allow other systems to monitor for changes */
+  if (utimensat (self->sysroot_fd, "ostree/deploy", NULL, 0) < 0)
+    {
+      glnx_set_prefix_error_from_errno (error, "%s", "futimens");
+      goto out;
+    }
+
   /* Now reload from disk */
   if (!ostree_sysroot_load (self, cancellable, error))
     {