futex: MAINTAINERS: Re-add selftests directory

Commit 95ca6d73

 ("docs: move locking-specific documents to
locking/") accidentally replaced the selftests line for a duplicated
documentation one. Revert this change.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Fixes: 95ca6d73 ("docs: move locking-specific documents to locking/")
Link: https://lore.kernel.org/r/20200715131036.9692-1-andrealmeid@collabora.com


Signed-off-by: Jonathan Corbet <corbet@lwn.net>