patch 8.0.0646: the hlsearch test fails on fast systems

Problem:    The hlsearch test fails on fast systems.
Solution:   Make the search pattern slower.  Fix that the old regexp engine
            doesn't timeout properly.
diff --git a/src/version.c b/src/version.c
index 2c3c45a..4683ba4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -765,6 +765,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    646,
+/**/
     645,
 /**/
     644,