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

glib/tests/test-printf.c

@@ -942,6 +942,7 @@ _Pragma ("GCC diagnostic pop")
 #endif
 }
 
+G_GNUC_PRINTF(1, 2)
 static gsize
 upper_bound (const gchar *format, ...)
 {