patch 8.0.0551: the typeahead buffer is reallocated too often

Problem:    The typeahead buffer is reallocated too often.
Solution:   Re-use the existing buffer if possible.

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    551,
 /**/
     550,
 /**/