user: MAINTAINERS: add cyphar as a maintainer

This patch adds Aleksa Sarai to the maintainer list for libcontainer's
user API.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
Merge pull request #294 from cyphar/user-add-cyphar-maintainer

user: MAINTAINERS: add cyphar (myself) as a maintainer