[OWNERS] Add joshuaseaton@ as a global approver

The infrastructure team makes varied changes across the tree, and so it
makes sense to have someone on the team as a global approver.

Change-Id: I198f3bfa477f80c4015c1de444d6fb71a0ff9333
diff --git a/OWNERS b/OWNERS
index c961d20..85ef6a9 100644
--- a/OWNERS
+++ b/OWNERS
@@ -9,6 +9,7 @@
 jamesr@google.com
 jeffbrown@google.com
 jeremymanson@google.com
+joshuaseaton@google.com
 mcgrathr@google.com
 mesch@google.com
 phosek@google.com