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.
diff --git a/src/version.c b/src/version.c
index aba7a80..2ea5708 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    551,
+/**/
     550,
 /**/
     549,