patch 7.4.1560

Problem:    Dict options with a dash are more difficult to use.
Solution:   Use an underscore, so that dict.err_io can be used.

src/version.c

@@ -743,6 +743,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1560,
 /**/
     1559,
 /**/