Update QGCTileCacheWorker.cpp

If TileDB is bigger than 2GB, 
--- sq = QString("SELECT COUNT(size), SUM(size) FROM Tiles WHERE tileID IN (SELECT A.tileID FROM SetTiles A join SetTiles B on A.tileID = B.tileID WHERE B.setID  ---
This query holds QGCTileCache Worker.
I solved problems with adding an index like that.

src/QtLocationPlugin/QGCTileCacheWorker.cpp

@@ -1061,6 +1061,8 @@ QGCCacheWorker::_createDB(QSqlDatabase* db, bool createDefault)
     {
         qWarning() << "Map Cache SQL error (create Tiles db):" << query.lastError().text();
     } else {
+        query.exec("CREATE INDEX hash ON Tiles ( hash, size, type ) ");
+             
         if(!query.exec(
             "CREATE TABLE IF NOT EXISTS TileSets ("
             "setID INTEGER PRIMARY KEY NOT NULL, "