Fix wxListCtrl sort callback signature in the test under Win64.

Use wxIntPtr instead of long which is not big enough to hold a pointer in
Win64 builds.

Closes #4309.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76994 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775