gtk_border_free is for freeing GtkBorder's, not g_free.

gtk_border_free is for freeing GtkBorder's, not g_free.
Using g_free instead used to have no ill effects as gtk_border_free called that anyway, but in gtk+-2.11 GtkBorder
uses GSlice and gtk_border_free therefore uses g_slice_free and using g_free makes things crash hard.
So fix it or wxGTK won't work with the upcoming gtk+-2.12 stable release planned for end of July.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@46513 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775