[gtk+] demos: Fix build failure

[Matthias Clasen <matthiasc src gnome org>]

commit 64739f46fc87a551d346267710423f14c9a78f2b
Author: Bastien Nocera <hadess hadess net>
Date:   Wed Jul 27 19:13:31 2016 +0200

    demos: Fix build failure
    
    gtk+/demos/gtk-demo/css_blendmodes.c: In function ‘update_css_for_blend_mode’:
    gtk+/demos/gtk-demo/css_blendmodes.c:49:26: error: format not a string literal, argument types not 
checked [-Werror=format-nonliteral]
                              blend_mode);
                              ^~~~~~~~~~
    
    https://bugzilla.gnome.org/show_bug.cgi?id=769236

 demos/gtk-demo/css_blendmodes.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/demos/gtk-demo/css_blendmodes.c b/demos/gtk-demo/css_blendmodes.c
index 4ce3836..dc41399 100644
--- a/demos/gtk-demo/css_blendmodes.c
+++ b/demos/gtk-demo/css_blendmodes.c
@@ -34,6 +34,8 @@ struct {
   { NULL }
 };
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
 static void
 update_css_for_blend_mode (GtkCssProvider *provider,
                            const gchar    *blend_mode)
@@ -53,6 +55,7 @@ update_css_for_blend_mode (GtkCssProvider *provider,
   g_bytes_unref (bytes);
   g_free (css);
 }
+#pragma GCC diagnostic pop
 
 static void
 row_activated (GtkListBox     *listbox,