QtSql: add missing key for MariaDB

When support for MariaDB was added in
947704cefe the key 'MARIADB' was not added
to the json file.

Change-Id: I8565a4a3804028806c1d2ff992329ece24de3fc5
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

src/plugins/sqldrivers/mysql/mysql.json

@@ -1,3 +1,3 @@
 {
-    "Keys": [ "QMYSQL3", "QMYSQL" ]
+    "Keys": [ "QMYSQL3", "QMYSQL", "QMARIADB" ]
 }