patch 8.2.1556: cursorline highlighting always overrules sign highlighting

Problem:    Cursorline highlighting always overrules sign highlighting.
Solution:   Combine the highlighting, use the priority to decide how.
            (closes #6812)

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1556,
 /**/
     1555,
 /**/