patch 8.1.1310: named function arguments are never optional

Problem:    Named function arguments are never optional.
Solution:   Support optional function arguments with a default value. (Andy
            Massimino, closes #3952)

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1310,
 /**/
     1309,
 /**/