verity: Update OWNERS file.

We'd like to have at least two active owners on each project.
Start by adding Mattias and we can later transition this to other
members of the security team.

BUG=None
TEST=None

Change-Id: I5979c8271a39e2a23363c6166cdd2ac1849a3c48
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dm-verity/+/1663998
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Commit-Queue: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/OWNERS b/OWNERS
index 3055a12..f3c03af 100644
--- a/OWNERS
+++ b/OWNERS
@@ -1,2 +1,2 @@
 jorgelo@chromium.org
-wad@chromium.org
+mnissler@chromium.org