[Helper] Add ejabberd to build overrides

helper-scripts/docker-compose.override.yml.d/BUILD_FLAGS/docker-compose.override.yml

@@ -43,3 +43,5 @@ services:
   olefy-mailcow:
     build: ./data/Dockerfiles/olefy
 
+  ejabberd-mailcow:
+    build: ./data/Dockerfiles/ejabberd