MAINTAINERS: add Davidlohr as GPT maintainer

Add a new entry for the GPT standard.  Any future changes will now be
routed through linux-efi.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Acked-by: Matt Fleming <matt.fleming@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Acked-by: Matt Domsch <Matt_Domsch@dell.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>