[libgxps] gxps-images: Fix non-literal format string warning for clang

[Carlos Garcia Campos <carlosgc src gnome org>]

commit 86414c28b633a4e0c729a66c486f523604411035
Author: Ting-Wei Lan <lantw src gnome org>
Date:   Wed Jan 6 03:03:00 2016 +0800

    gxps-images: Fix non-literal format string warning for clang
    
    https://bugzilla.gnome.org/show_bug.cgi?id=760177

 libgxps/gxps-images.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/libgxps/gxps-images.c b/libgxps/gxps-images.c
index 7d62508..1ced845 100644
--- a/libgxps/gxps-images.c
+++ b/libgxps/gxps-images.c
@@ -608,6 +608,7 @@ fill_tiff_error (GError     **error,
        }
 }
 
+G_GNUC_PRINTF (2, 0)
 static void
 _tiff_error_handler (const char *mod,
                     const char *fmt,