games/gtkradiant: unbreak with libc++ 8

In file included from plugins/archivepak/plugin.cpp:24:
In file included from include/iarchive.h:25:
In file included from /usr/include/c++/v1/cstddef:38:
include/version:1:1: error: expected unqualified-id
1.5.0
^

PR: 236192
Approved by: portmgr blanket