patch 9.1.0018: use of #if instead of #ifdef

Problem:  use of #if instead of #ifdef
Solution: use correct form of #ifdef

`#if FEAT_GUI_HAIKU` was used mistakenly. Use the correct form
`#ifdef FEAT_GUI_HAIKU` instead.

closes: #13843

Signed-off-by: Ken Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/gui.h b/src/gui.h
index 0a1d274..1882000 100644
--- a/src/gui.h
+++ b/src/gui.h
@@ -176,7 +176,7 @@
 				// to reduce the count.
 #endif
 
-#if FEAT_GUI_HAIKU
+#ifdef FEAT_GUI_HAIKU
     VimScrollBar *id;		// Pointer to real scroll bar
 #endif
 #ifdef FEAT_GUI_PHOTON