use PLUGIN_LICENSE_GPL as plugin license instead of PLUGIN_LICENSE_PR…

…OPRIETARY.

Groonga storage engine is released under the LGPL v2 or later but it uses
MySQL that is licensed under GPL v2 only. So users will use groonga storage
engine as GPL v2.

Suggested by Kazuhiko. Thanks!!!

ha_mroonga.cc

@@ -388,7 +388,7 @@ struct st_mysql_plugin i_s_groonga_stats =
   "groonga_stats",
   plugin_author,
   "Statistics for groonga",
-  0,
+  PLUGIN_LICENSE_GPL,
   i_s_groonga_stats_init,
   i_s_groonga_stats_deinit,
   0x0001,
@@ -1115,7 +1115,7 @@ mysql_declare_plugin(mroonga)
   "groonga",
   "Tetsuro IKEDA",
   "Fulltext search, column base",
-  0,
+  PLUGIN_LICENSE_GPL,
   mrn_init,
   mrn_deinit,
   MRN_VERSION_IN_HEX,