Avoid unused variable warning in wxPropertyGrid code in non-wxGTK.

Put the variable only used in wxGTK inside "#ifdef __WXGTK__".

This fixes the warning but the real fix would be to get rid of this variable
completely and just fix whatever problem in wxWindow::Navigate() this was
supposed to work around.

See #14459.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71973 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/propgrid/propgrid.cpp

@@ -5553,12 +5553,14 @@ void wxPropertyGrid::HandleKeyEvent( wxKeyEvent &event, bool fromChild )
 
     if ( keycode == WXK_TAB )
     {
+#if defined(__WXGTK__)
         wxWindow* mainControl;
 
         if ( HasInternalFlag(wxPG_FL_IN_MANAGER) )
             mainControl = GetParent();
         else
             mainControl = this;
+#endif
 
         if ( !event.ShiftDown() )
         {