Add OWNERS file for external/ninja that references build/soong/OWNERS

Test: treehugger
Bug: 170407947
Change-Id: I3f380fc19bb6a99561b7c7139275525fcd4b4531
diff --git a/OWNERS b/OWNERS
new file mode 100644
index 0000000..4cac0f5
--- /dev/null
+++ b/OWNERS
@@ -0,0 +1 @@
+include platform/build/soong:/OWNERS