patch 9.0.1554: code for handling 'switchbuf' is repeated

Problem:    Code for handling 'switchbuf' is repeated.
Solution:   Add a function to handle 'switchbuf'. (Yegappan Lakshmanan,
            closes #12397)

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1554,
 /**/
     1553,
 /**/