Update OWNERS for external/eigen.

Test: n/a
Change-Id: I3bc647c29c93d08ef7d8d896a114bbdb398103c6
diff --git a/OWNERS b/OWNERS
index acd6ec0..e77f139 100644
--- a/OWNERS
+++ b/OWNERS
@@ -1,2 +1,3 @@
 miaowang@google.com
 timmurray@google.com
+ianhua@google.com