[gnome-boxes] Update help_linguas

[Piotr Drąg <piotrdrag src gnome org>]

commit 2a3b78d5c59a3b5ece5b933b4d28d84e0fdc10f2
Author: Piotr Drąg <piotrdrag gmail com>
Date:   Sat Dec 9 23:00:12 2017 +0100

    Update help_linguas

 help/meson.build |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/help/meson.build b/help/meson.build
index 09aa9f2..8d5666b 100644
--- a/help/meson.build
+++ b/help/meson.build
@@ -41,6 +41,7 @@ help_linguas = [
   'hu',
   'it',
   'ko',
+  'pl',
   'pt_BR',
   'sv'
 ]