MAINTAINERS: Fix ANDROID AB unknown file entry

Commit 19a91f2464a8 ("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: 19a91f2464a8 ("Create a new boot/ directory")
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 9695144..1e9a9ee 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -60,8 +60,8 @@
 M:	Igor Opaniuk <igor.opaniuk@gmail.com>
 R:	Sam Protsenko <semen.protsenko@linaro.org>
 S:	Maintained
+F:	boot/android_ab.c
 F:	cmd/ab_select.c
-F:	common/android_ab.c
 F:	doc/android/ab.rst
 F:	include/android_ab.h
 F:	test/py/tests/test_android/test_ab.py