[autotests] Only use QTest::setMainSourcePath if we have Qt 5.5

Update to f5fbdaa6ca which didn't solve
the problem. Seems to be new function in Qt 5.5.

autotests/wayland/kwin_wayland_test.h

@@ -60,7 +60,9 @@ int main(int argc, char *argv[]) \
     KWin::WaylandTestApplication app(argc, argv); \
     app.setAttribute(Qt::AA_Use96Dpi, true); \
     TestObject tc; \
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 5, 0)) \
     QTest::setMainSourcePath("anonymous"); \
+#endif \
     return QTest::qExec(&tc, argc, argv); \
 }