Fixed the undefined reference to pow error

That [fixes](https://travis-ci.org/nurupo/InsertProjectNameHere/jobs/26461313) the [pow error](https://travis-ci.org/nurupo/ProjectTox-Qt-GUI/jobs/26300122) on Travis with gcc-4.8 for me, though one of tests seems to be timing out constantly
```
../auto_tests/tox_test.c:317:E:many_clients:test_many_clients:0: (after this point) Test timeout expired
```