patch 8.0.1415: warning for unused function without timers feature

Problem:    Warning for unused function without timers feature.
Solution:   Add #ifdef. (John Marriott)
diff --git a/src/gui.c b/src/gui.c
index 916c9be..3456f92 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -2885,6 +2885,7 @@
     }
 }
 
+#ifdef FEAT_TIMERS
 /*
  * Passed to ui_wait_for_chars_or_timer(), ignoring extra arguments.
  */
@@ -2896,6 +2897,7 @@
 {
     return gui_mch_wait_for_chars(wtime);
 }
+#endif
 
 /*
  * Returns OK if a character was found to be available within the given time,