patch 8.2.0320: no Haiku support

Problem:    No Haiku support.
Solution:   Add support for Haiku. (Emir Sari, closes #5605)
diff --git a/src/mouse.c b/src/mouse.c
index 5b468d6..5abb942 100644
--- a/src/mouse.c
+++ b/src/mouse.c
@@ -538,7 +538,7 @@
 		    // menu on the button down event.
 		    return FALSE;
 #  endif
-#  if defined(FEAT_GUI_ATHENA) || defined(FEAT_GUI_MSWIN)
+#  if defined(FEAT_GUI_ATHENA) || defined(FEAT_GUI_MSWIN) || defined(FEAT_GUI_HAIKU)
 		if (is_click || is_drag)
 		    // Ignore right button down and drag mouse events.  Windows
 		    // only shows the popup menu on the button up event.