patch 9.0.1581: translation does not work for plural argument

Problem:    Translation does not work for plural argument.
Solution:   Use PLURAL_MSG() for errors and with xgettext. (closes #12443)

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1581,
 /**/
     1580,
 /**/