Use G_GNUC_PRINTF() for simple_message_box().

We don't need to do the right __attribute__ magic ourselves; GLib does
it for us.

Change-Id: Id31943a8413f0272237be4f868659b5bd5e70626
Reviewed-on: https://code.wireshark.org/review/2758
Reviewed-by: Guy Harris <guy@alum.mit.edu>