patch 9.0.1537: message for opening the cmdline window is not translated

Problem:    Message for opening the cmdline window is not translated.
Solution:   Add gettext() and scan the defaults script for text to be
            translated.  (closes #12371)
diff --git a/runtime/defaults.vim b/runtime/defaults.vim
index f1d5cd1..f449c6e 100644
--- a/runtime/defaults.vim
+++ b/runtime/defaults.vim
@@ -120,7 +120,7 @@
     au!
     autocmd CmdwinEnter *
 	  \ echohl Todo | 
-	  \ echo 'You discovered the command-line window! You can close it with ":q".' |
+	  \ echo gettext('You discovered the command-line window! You can close it with ":q".') |
 	  \ echohl None
   augroup END