Add OWNERS in external/avb

Owners are selected from top CL approvals or owners.
They will be suggested to review/approve future CLs.

Test: build/make/tools/checkowners.py -c -v OWNERS
Change-Id: Id2354ccfc7fbe07d13da4f9b5fd5338af7615ed5
diff --git a/OWNERS b/OWNERS
new file mode 100644
index 0000000..b93049c
--- /dev/null
+++ b/OWNERS
@@ -0,0 +1,4 @@
+bowgotsai@google.com
+samitolvanen@google.com
+zeuthen@google.com
+dkrahn@google.com