Suppress harmless mingw32 warning about unused value.

Add an explicit cast to void to Header_SetImageList() call to avoid "warning:
value computed is not used".

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62038 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/headerctrl.cpp

@@ -285,6 +285,7 @@ void wxHeaderCtrl::DoInsertItem(const wxHeaderColumn& col, unsigned int idx)
             if ( !m_imageList )
             {
                 m_imageList = new wxImageList(bmpWidth, bmpHeight);
+                (void) // suppress mingw32 warning about unused computed value
                 Header_SetImageList(GetHwnd(), GetHimagelistOf(m_imageList));
             }
             else // already have an image list