patch 8.1.0574: 'commentstring' not used when adding fold marker in C

Problem:    'commentstring' not used when adding fold marker in C.
Solution:   Require white space before middle comment part. (mostly by
            Hirohito Higashi)

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    574,
 /**/
     573,
 /**/