Let windows.c actually get the tox *m.

main.c called init_windows(m), but windows.c only had init_windows(). This caused m to be NULL, which didn't cause any crashes, but it was certainly a headache for trying to do some new stuff.

For instance I was calling tox_addfriend_norequest() in windows.c:on_request() to auto-add friends. It kept crashing on me since the global var Tox *m was never assigned anything.