textproc/groonga: unbreak with libc++ 8

In file included from cursor-factory.cpp:18:
In file included from ./cursor-factory.hpp:21:
In file included from ./cursor.hpp:21:
In file included from ./key.hpp:21:
In file included from ./string.hpp:21:
In file included from ./dat.hpp:26:
In file included from /usr/include/c++/v1/cstddef:38:
../../version:1:1: error: expected unqualified-id
9.0.0
^

PR: 236192
Approved by: portmgr blanket