patch 8.1.2247: "make vimtags" does not work in runtime/doc

Problem:    "make vimtags" does not work in runtime/doc.
Solution:   Test existence with "which" instead of "test -x". (Ken Takata)

src/version.c

@@ -741,6 +741,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2247,
 /**/
     2246,
 /**/