MAINTAINERS: add Aleksa Sarai to maintainers

In light of my recent contributions, I believe myself to be technically
competent enough to be a maintainer of runC. I'm an active contributor,
authored the PIDs cgroup subsystem specfically for the runC and Docker
projects, and am already a maintainer of libcontainer/user.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

MAINTAINERS

@@ -6,3 +6,4 @@ Alexander Morozov <lk4d4@docker.com> (@LK4D4)
 Daniel, Dao Quang Minh <dqminh89@gmail.com> (@dqminh)
 Andrey Vagin <avagin@virtuozzo.com> (@avagin)
 Qiang Huang <h.huangqiang@huawei.com> (@hqhq)
+Aleksa Sarai <asarai@suse.de> (@cyphar)