patch 8.1.0835: GUI build fails on MS-Windows

Problem:    GUI build fails on MS-Windows.
Solution:   Adjust #ifdef.
diff --git a/src/ui.c b/src/ui.c
index 0d08c5d..d4278a3 100644
--- a/src/ui.c
+++ b/src/ui.c
@@ -399,7 +399,7 @@
 #endif
 
 	if ((resize_func != NULL && resize_func(TRUE))
-#ifdef FEAT_CLIENTSERVER
+#if defined(FEAT_CLIENTSERVER) && defined(UNIX)
 		|| server_waiting()
 #endif
 #ifdef MESSAGE_QUEUE