tst_QSqlThread: match #if-ery on declaration to use

Variable was declared outside #if-ery but only used inside,
leading to warnings when the define wasn't set.

Change-Id: Iad6a236bdf76b8ca0a58a84509d19f7e1a3e2df2
Reviewed-by: Mikhail Svetkin <mikhail.svetkin@qt.io>
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

tests/auto/sql/kernel/qsqlthread/tst_qsqlthread.cpp

@@ -401,8 +401,10 @@ void tst_QSqlThread::readWriteThreading()
     QTRY_VERIFY_WITH_TIMEOUT(threadFinishedCount >= 2, 10000);
 }
 
+#ifdef QOCI_THREADED
 // run with n threads in parallel. Change this constant to hammer the poor DB server even more
 static const int maxThreadCount = 4;
+#endif
 
 void tst_QSqlThread::readFromSingleConnection()
 {