MAINTAINERS: Fix ANDROID AB unknown file entry

Commit 19a91f24 ("Create a new boot/ directory") moved the
android_ab.c code under boot/android_ab but did not update
the MAINTAINERS entry.

Update it so that the maintainer will get cc'ed again.

Fixes: 19a91f24 ("Create a new boot/ directory")
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>