Fix problems due to using "%i" format with size_t in propgrid sample.

Cast size_t value to int when passing it to "%i" format specifier as it has a
different size on LP64 platforms.

Closes #16213.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76423 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775