Ensure Qt builds with -no-feature-shortcut on Windows

Building without shortcuts would fail due to a missing QKeySequence.
This was already handled in the implementation but required an ifdef in
the header.

Change-Id: I12c92ca480d0abdb518aae00b5b7e259133ef9ca
Pick-to: 6.5 6.8 6.9
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/plugins/platforms/windows/qwindowsmenu.h

@@ -76,7 +76,9 @@ private:
     bool m_checkable = false;
     bool m_checked = false;
     bool m_enabled = true;
+#if QT_CONFIG(shortcut)
     QKeySequence m_shortcut;
+#endif
 };
 
 class QWindowsMenu : public QPlatformMenu