Use connection name to avoid closing default connection

No observable bad effects identified but the following warning looks suspicious:

  Warning: QSqlDatabasePrivate::addDatabase: duplicate connection name qt_sql_default_connection, old connection removed.

Author: m-kuhn

src/core/qgsuserprofile.cpp

@@ -67,7 +67,7 @@ const QString QgsUserProfile::alias() const
     return name();
   }
 
-  QSqlDatabase db = QSqlDatabase::addDatabase( QStringLiteral( "QSQLITE" ) );
+  QSqlDatabase db = QSqlDatabase::addDatabase( QStringLiteral( "QSQLITE" ), QStringLiteral( "userprofile" ) );
   db.setDatabaseName( qgisDB() );
   if ( !db.open() )
     return name();