Add G_GNUC_PRINTF on all functions with format strings

This allows compilation with clang without errors, even when
-Wformat-nonliteral is active (as long as there are no real cases of
non literal formatting).

https://bugzilla.gnome.org/show_bug.cgi?id=691608

gio/gdbusauthmechanismsha1.c

@@ -469,6 +469,7 @@ keyring_lookup_entry (const gchar  *cookie_context,
 }
 
 /* function for logging important events that the system administrator should take notice of */
+G_GNUC_PRINTF(1, 2)
 static void
 _log (const gchar *message,
       ...)