releasetools: Update OWNERS.

Test: N/A
Change-Id: I0c6ac3cd04cc6034787d4b8b90fbd61349c54655
diff --git a/tools/releasetools/OWNERS b/tools/releasetools/OWNERS
index 39448cf..766adb4 100644
--- a/tools/releasetools/OWNERS
+++ b/tools/releasetools/OWNERS
@@ -1 +1,2 @@
 tbao@google.com
+xunchang@google.com