copywrite: add placeholder for OSS/ENT ignore split. (#17965)

.copywrite.hcl

@@ -7,5 +7,8 @@ project {
   header_ignore = [
     "command/asset/*.hcl",
     "command/agent/bindata_assetfs.go",
+    // Enterprise files do not fall under the open source licensing. OSS-ENT
+    // merge conflicts might happen here, please be sure to put new OSS
+    // exceptions above this comment.
   ]
 }