fix building on IRIX with GCC
It's impossible to know what problem this was supposed to fix (it's been there
since the Dawn of Time: r2), but it prevents stddef.h from defining ptrdiff_t,
among others. We need ptrdiff_t.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76488 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775