MDEV-7604: wsrep plugin lists its plugin_maturity as Unknown

Changed wsrep plugin from mysql_plugin to maria_plugin to
set up plugin_maturity to Stable.

mysql-test/suite/wsrep/r/plugin.result

+SELECT plugin_name,plugin_version,plugin_maturity   FROM information_schema.plugins    where plugin_name like 'wsrep' ORDER BY plugin_maturity,plugin_name;
+plugin_name	plugin_version	plugin_maturity
+wsrep	1.0	Stable

mysql-test/suite/wsrep/t/plugin.test

+--source include/have_wsrep.inc
+
+#
+# MDEV-7604: wsrep plugin lists its status as Unknown
+#
+
+SELECT plugin_name,plugin_version,plugin_maturity   FROM information_schema.plugins    where plugin_name like 'wsrep' ORDER BY plugin_maturity,plugin_name;
\ No newline at end of file

sql/wsrep_hton.cc

@@ -561,7 +561,7 @@ struct st_mysql_storage_engine wsrep_storage_engine=
 { MYSQL_HANDLERTON_INTERFACE_VERSION };
 
 
-mysql_declare_plugin(wsrep)
+maria_declare_plugin(wsrep)
 {
   MYSQL_STORAGE_ENGINE_PLUGIN,
   &wsrep_storage_engine,
@@ -575,7 +575,7 @@ mysql_declare_plugin(wsrep)
   0x0100 /* 1.0 */,
   NULL,                       /* status variables                */
   NULL,                       /* system variables                */
-  NULL,                       /* config options                  */
-  0,                          /* flags                           */
+  "1.0",         /* string version */
+  MariaDB_PLUGIN_MATURITY_STABLE /* maturity */
 }
-mysql_declare_plugin_end;
+maria_declare_plugin_end;