MAINTAINERS: add Rodrigo Campos

I am nominating @rata for the role of runc maintainer.

He is pretty active in the project, did some substantial work
and is helping with PR review and releases.

As noted in MAINTAINERS_GUIDE.md, we have a week to vote, and need to
get 66% of current maintainers' votes.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

MAINTAINERS

@@ -7,3 +7,4 @@ Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp> (@AkihiroSuda)
 Kir Kolyshkin <kolyshkin@gmail.com> (@kolyshkin)
 Sebastiaan van Stijn <github@gone.nl> (@thaJeztah)
 Li Fu Bang <lifubang@acmcoder.com> (@lifubang)
+Rodrigo Campos <rodrigoca@microsoft.com> (@rata)