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/version.c b/src/version.c
index f440140..675103f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    18,
+/**/
     17,
 /**/
     16,