MAINTAINERS: update Kselftest Framework mailing list

Kselftest Framework now has a dedicated mailing list linux-kselftest.
Update the entry in MAINTAINERS file.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>