[metacity] libmetacity: fix build warning

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 698124215dc3d3bdca38aa09fadabf79820f015e
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Tue Feb 23 03:47:46 2016 +0200

    libmetacity: fix build warning

 libmetacity/meta-theme-metacity.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/libmetacity/meta-theme-metacity.c b/libmetacity/meta-theme-metacity.c
index 639ffef..92e87fd 100644
--- a/libmetacity/meta-theme-metacity.c
+++ b/libmetacity/meta-theme-metacity.c
@@ -279,6 +279,7 @@ peek_required_version (ParseInfo *info)
     return info->metacity->format_version;
 }
 
+G_GNUC_PRINTF(5, 6)
 static void
 set_error (GError              **err,
            GMarkupParseContext  *context,