ICC: Fix MySQL driver build

In function `QMYSQLDriver::formatValue(QSqlField const&, bool) const':
qsql_mysql.cpp:1569: undefined reference to `QString::operator=(
QLatin1String)

Change-Id: I56104cdd53fdc083670510f24b735cf05948f156
Reviewed-by: Samuel Gaist <samuel.gaist@edeltech.ch>
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/sql/drivers/mysql/qsql_mysql.cpp

@@ -1566,7 +1566,7 @@ QString QMYSQLDriver::formatValue(const QSqlField &field, bool trimStrings) cons
     Q_D(const QMYSQLDriver);
     QString r;
     if (field.isNull()) {
-        r = QLatin1String("NULL");
+        r = QStringLiteral("NULL");
     } else {
         switch(field.type()) {
         case QVariant::Double: