timer_alarm: Set fd to -1 to avoid warnings on tgl_timer_delete

Warnings like this:

GLib-CRITICAL **: Source ID N was not found when attempting to remove it

tgp-timers.c

@@ -31,6 +31,7 @@ struct tgl_timer {
 
 static int timer_alarm (gpointer arg) {
   struct tgl_timer *t = arg;
+  t->fd = -1;
   t->cb (t->TLS, t->arg);
   return FALSE;
 }