patch 8.2.0671: Haiku: compiler warnings

Problem:    Haiku: compiler warnings.
Solution:   Avoid the warnings. Drop display_errors() copy. (Emir Sari,
            closes #6018)
diff --git a/src/gui.c b/src/gui.c
index 5959043..13b65cb 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -5107,7 +5107,8 @@
 # endif
 #endif
 
-#if defined(FEAT_GUI_GTK) || defined(FEAT_GUI_X11) || defined(PROTO)
+#if defined(FEAT_GUI_GTK) || defined(FEAT_GUI_X11)|| defined(FEAT_GUI_HAIKU) \
+	|| defined(PROTO)
     void
 display_errors(void)
 {