[totem] build: Fix error before error when vala is missing

[Bastien Nocera <hadess src gnome org>]

commit 7f54e1f0369cf2087655ab3e7fd071440d3f867f
Author: Bastien Nocera <hadess hadess net>
Date:   Wed Jun 27 15:41:38 2018 +0200

    build: Fix error before error when vala is missing
    
    Instead of:
    str = 'you need vala ' + vala_req_version + ' installed to use vala plugins'
    you'd get:
    meson.build:225:6: ERROR:  Unknown variable "vala_req_version".
    because vala_req_version isn't defined in the right scope.

 meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/meson.build b/meson.build
index 47b0cb84..3653e92c 100644
--- a/meson.build
+++ b/meson.build
@@ -214,8 +214,8 @@ have_vala = false
 vala_option = get_option('enable-vala')
 if vala_option != 'no'
   if have_gir
+    vala_req_version = '>= 0.14.1'
     if add_languages('vala', required: false)
-      vala_req_version = '>= 0.14.1'
       if meson.get_compiler('vala').version().version_compare(vala_req_version)
         have_vala = true
       endif