[build] Update //build/OWNERS

juliehockett@ has left the team - and joshuaseaton@ makes varied
build changes with relative frequency.

Change-Id: Icd7cf242a4950d24a1f0c2e2827317e55663fd89
diff --git a/build/OWNERS b/build/OWNERS
index f24dba3..632cd8d 100644
--- a/build/OWNERS
+++ b/build/OWNERS
@@ -4,7 +4,7 @@
 pylaligand@google.com
 raggi@google.com
 abarth@google.com
-juliehockett@google.com
+joshuaseaton@google.com
 per-file BUILD.gn=jseibert@google.com
 per-file BUILD.gn=bryanhenry@google.com
 per-file BUILD.gn=zarvox@google.com