games/species: Fix build on 12.0-CURRENT

kingdom.cpp:149:19: error: assigning to 'bool' from incompatible type 'std::__1::fstream' (aka 'basic_fstream<char>')

_in_experiment = fs;
                 ^~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p475478_s336801/logs/errors/species-1.2d_1.log