sysutils/lineakd: Fix build with Clang 6

/usr/include/c++/v1/map:629:15: error: object of type 'std::1::pair<const std::1::basic_string<char>, std::1::vector<keycommand_info, std::1::allocator<keycommand_info> > >' cannot be assigned because its copy assignment operator is implicitly deleted

{__nc = __v.__cc; return *this;}
      ^

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p475964_s336959/logs/errors/lineakd-0.9_4.log