MAINTAINERS: add Josh Poimboeuf as faddr2line maintainer

... so I finally get credit for my greatest accomplishment.

And, less importantly, so get_maintainer.pl will actually CC me on
future patches.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>