tst_qstandardpaths: #if-out a function only used within #if-ery

Avoids an unused function warning.

Change-Id: Id221595920e9a34eb83b66fe123d664f60fcae05
Reviewed-by: Mikhail Svetkin <mikhail.svetkin@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

tests/auto/corelib/io/qstandardpaths/tst_qstandardpaths.cpp

@@ -183,12 +183,14 @@ void tst_qstandardpaths::testDefaultLocations()
 #endif
 }
 
+#ifdef Q_XDG_PLATFORM
 static void createTestFile(const QString &fileName)
 {
     QFile file(fileName);
     QVERIFY(file.open(QIODevice::WriteOnly));
     QVERIFY(file.write("Hello"));
 }
+#endif
 
 void tst_qstandardpaths::testCustomLocations()
 {