[rygel] external: Make constant string const

[Jens Georg <jensgeorg src gnome org>]

commit a4caf3cdf011deda73a0c2e3496d7d9c259d9115
Author: Jens Georg <mail jensge org>
Date:   Wed Nov 30 21:02:57 2016 +0100

    external: Make constant string const
    
    Otherwise build fails against recent vala
    
    Signed-off-by: Jens Georg <mail jensge org>

 .../external/rygel-external-variant-util.vala      |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/plugins/external/rygel-external-variant-util.vala 
b/src/plugins/external/rygel-external-variant-util.vala
index fa4da04..15194d8 100644
--- a/src/plugins/external/rygel-external-variant-util.vala
+++ b/src/plugins/external/rygel-external-variant-util.vala
@@ -19,7 +19,7 @@
  */
 
 namespace Rygel.External {
-    public static string MANDATORY_MISSING_MESSAGE =
+    public const string MANDATORY_MISSING_MESSAGE =
         N_("External provider %s did not provide mandatory property “%s”");
 
     public static Variant? get_mandatory