patch 8.1.1403: cannot build without the timer feature

Problem:    Cannot build without the timer feature.
Solution:   Add #ifdef.
diff --git a/src/structs.h b/src/structs.h
index 7ead00c..ca7c540 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2874,7 +2874,9 @@
     int		w_zindex;
     int		w_maxheight;	    // "maxheight" for popup window
     int		w_maxwidth;	    // "maxwidth" for popup window
+# if defined(FEAT_TIMERS)
     timer_T	*w_popup_timer;	    // timer for closing popup window
+# endif
 #endif