MAINTAINERS: add qemu_vga.ndrv file entry for Mac machines

The VGA driver built from Ben's QemuMacDrivers repository is used exclusively
by the Mac machines.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>