patch 8.0.1621: using invalid default value for highlight attribute

Problem:    Using invalid default value for highlight attribute.
Solution:   Use zero instead of -1.

src/version.c

@@ -766,6 +766,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1621,
 /**/
     1620,
 /**/