Add sethmo@google.com to system/security/OWNERS

Test: n/a
Change-Id: I28d293a51dd4dda8ff003a946e66785c1f66c3ce
diff --git a/OWNERS b/OWNERS
index daf2f93..93c024d 100644
--- a/OWNERS
+++ b/OWNERS
@@ -3,5 +3,6 @@
 jbires@google.com
 jdanis@google.com
 kroot@google.com
+sethmo@google.com
 swillden@google.com
 zeuthen@google.com