Build: Set xpack to require keystore

See https://github.com/elastic/elasticsearch/pull/26329

Original commit: elastic/x-pack-elasticsearch@e77361a6d5

plugin/build.gradle

@@ -14,6 +14,7 @@ esplugin {
   description 'Elasticsearch Expanded Pack Plugin'
   classname 'org.elasticsearch.xpack.XPackPlugin'
   hasNativeController true
+  requiresKeystore true
   licenseFile project(':x-pack-elasticsearch').file('LICENSE.txt')
   noticeFile project(':x-pack-elasticsearch').file('NOTICE.txt')
 }