math/gap: unbreak with libc++ 8

In file included from src/bistellar_move.cpp:9:
In file included from src/bistellar_move.h:12:
In file included from /usr/include/c++/v1/iostream:38:
In file included from /usr/include/c++/v1/ios:216:
In file included from /usr/include/c++/v1/__locale:15:
In file included from /usr/include/c++/v1/string:505:
In file included from /usr/include/c++/v1/string_view:176:
In file included from /usr/include/c++/v1/__string:57:
In file included from /usr/include/c++/v1/algorithm:640:
In file included from /usr/include/c++/v1/initializer_list:47:
In file included from /usr/include/c++/v1/cstddef:38:
./version:1:1: error: expected unqualified-id
2.1.7
^

PR: 236192
Approved by: portmgr blanket