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/gdbusauth.c

@@ -46,6 +46,7 @@
 
 #include "glibintl.h"
 
+G_GNUC_PRINTF(1, 2)
 static void
 debug_print (const gchar *message, ...)
 {