Add @mxinden to CODEOWNERS (#2025)

Discussed yesterday with @KershawChang. @martinthomson, are you OK with this? @mxinden is pretty actively reviewing, so it might unblock getting PRs merged when both of you are busy.

Signed-off-by: Lars Eggert <lars@eggert.org>

.github/CODEOWNERS

@@ -1 +1 @@
-*    @KershawChang @martinthomson @larseggert
+*    @KershawChang @martinthomson @larseggert @mxinden