Update CODEOWNERS

Ensure no-one can add to owners other than myself as the official Google representative of this repo.

.github/CODEOWNERS

diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
index 5436e2c..bc0b315 100644
--- a/.github/CODEOWNERS
+++ b/.github/CODEOWNERS
@@ -1,2 +1,5 @@
 # Default owner
 * @aardappel @dbaileychess derekbailey@google.com
+
+# Prevent modification of this file
+.github/CODEOWNERS @dbaileychess derekbailey@google.com