Use new Qt flag to disable high DPI scaling on X

This flag was set for main_wayland but not X.

BUG: 357896
REVIEW: 126810

main_x11.cpp

@@ -269,6 +269,9 @@ KWIN_EXPORT int kdemain(int argc, char * argv[])
     setenv("QT_QPA_PLATFORM", "xcb", true);
 
     qunsetenv("QT_DEVICE_PIXEL_RATIO");
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 6, 0))
+    QCoreApplication::setAttribute(Qt::AA_DisableHighDpiScaling);
+#endif
 
     KWin::ApplicationX11 a(argc, argv);
     a.setupTranslator();