patch 8.1.2006: build failure with huge features but without channel feature

Problem:    Build failure with huge features but without channel feature.
Solution:   Add #ifdef. (Dominique Pelle, closes #4906)
diff --git a/src/ui.c b/src/ui.c
index 1b700f1..75b66fe 100644
--- a/src/ui.c
+++ b/src/ui.c
@@ -479,7 +479,9 @@
 	    // There is a pending job or channel, should return soon in order
 	    // to handle them ASAP.  Do check for input briefly.
 	    due_time = 10L;
+#  ifdef FEAT_JOB_CHANNEL
 	    brief_wait = TRUE;
+#  endif
 	}
 # endif
 	if (wait_func(due_time, interrupted, ignore_input))