Mark x-pack plugin as having native controller

This commit marks the x-pack plugin as having a native controller. This
is now a requirement in core for any plugin that forks a native process
to display a warning to the user when they install the plugin.

Relates elastic/x-pack-elasticsearch#839

Original commit: elastic/x-pack-elasticsearch@3529250023

plugin/build.gradle

@@ -24,6 +24,7 @@ esplugin {
   name 'x-pack'
   description 'Elasticsearch Expanded Pack Plugin'
   classname 'org.elasticsearch.xpack.XPackPlugin'
+  hasNativeController true
   licenseFile project(':x-pack-elasticsearch').file('LICENSE.txt')
   noticeFile project(':x-pack-elasticsearch').file('NOTICE.txt')
 }