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)
diff --git a/src/version.c b/src/version.c
index da4eed7..330914d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1581,
+/**/
     1580,
 /**/
     1579,