MDEV-7519 debian / ubuntu packaging creation of plugin table (if not exists)

Previously IF NOT EXISTS was added in Debian version in revno 3543. 
Now added the same for Ubuntu.

debian/dist/Ubuntu/mariadb-server-5.5.postinst

@@ -213,7 +213,7 @@ EOF
     # admin might already have chosen to remove one or more plugins. Newlines are necessary.
     install_plugins=`/bin/echo -e \
         "USE mysql;\n" \
-        "CREATE TABLE plugin (name char(64) COLLATE utf8_bin NOT NULL DEFAULT '', " \
+        "CREATE TABLE IF NOT EXISTS plugin (name char(64) COLLATE utf8_bin NOT NULL DEFAULT '', " \
         "  dl char(128) COLLATE utf8_bin NOT NULL DEFAULT '', " \
         "  PRIMARY KEY (name)) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin COMMENT='MySQL plugins';" `