Compilation fix for TempHWNDSetter in wxQt

Fix wxQt build after 2aef6570bb:
TempHWNDSetter can only be defined when using wxMSW.

include/wx/msw/private.h

@@ -374,6 +374,12 @@ inline RECT wxGetClientRect(HWND hwnd)
 // small helper classes
 // ---------------------------------------------------------------------------
 
+// This class can only be used with wxMSW wxWindow, as it doesn't have
+// {Set,Get}HWND() methods in the other ports, but this file is currently
+// included for wxQt/MSW too. It's not clear whether it should be, really, but
+// for now allow it to compile in this port too.
+#ifdef __WXMSW__
+
 // Temporarily assign the given HWND to the window in ctor and unset it back to
 // the original value (usually 0) in dtor.
 class TempHWNDSetter
@@ -397,6 +403,8 @@ private:
     wxDECLARE_NO_COPY_CLASS(TempHWNDSetter);
 };
 
+#endif // __WXMSW__
+
 // create an instance of this class and use it as the HDC for screen, will
 // automatically release the DC going out of scope
 class ScreenHDC