patch 8.2.1336: build failure on non-Unix systems

Problem:    Build failure on non-Unix systems.
Solution:   Add #ifdef.
diff --git a/src/gui.c b/src/gui.c
index e474554..5d003a3 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -5588,7 +5588,10 @@
     int c = merge_modifyOtherKeys(key, &modifiers);
 
     if ((c == Ctrl_C && ctrl_c_interrupts)
-	    || (intr_char != Ctrl_C && c == intr_char))
+#ifdef UNIX
+	    || (intr_char != Ctrl_C && c == intr_char)
+#endif
+	    )
     {
 	got_int = TRUE;
 	return c;