mail/notmuch: unbreak with libc++ 8

In file included from lib/database.cc:21:
In file included from ./lib/database-private.h:39:
In file included from /usr/local/include/xapian.h:56:
In file included from /usr/local/include/xapian/error.h:30:
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
0.28.1
^

PR: 236192
Approved by: portmgr blanket