QWindows(XP)Style: fix build with Clang after includemocs run

Clang doesn't like the unused member variable:

  qwindowsstyle_p.h💯11: error: private field 'reserved' is not used [-Werror,-Wunused-private-field]

Remove. It's private API.

Triggered by Clang seeing all methods of the classes in
one TU by the following includemocs commit.

Change-Id: I84e92d63af573c090ef89c1d8ee19af30f90b171
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/widgets/styles/qwindowsstyle_p.h

@@ -97,7 +97,6 @@ protected:
 private:
     Q_DISABLE_COPY(QWindowsStyle)
     Q_DECLARE_PRIVATE(QWindowsStyle)
-    void *reserved;
 };
 
 #endif // QT_NO_STYLE_WINDOWS

src/widgets/styles/qwindowsxpstyle_p.h

@@ -93,7 +93,6 @@ private:
     Q_DISABLE_COPY(QWindowsXPStyle)
     Q_DECLARE_PRIVATE(QWindowsXPStyle)
     friend class QStyleFactory;
-    void *reserved;
 };
 
 #endif // QT_NO_STYLE_WINDOWSXP