MixStream: g_thread_init is deprecated

[Linkid]

This warning was raised when compiling the MixStream extension:

fretwork/mixstream/MixStream.c: In function ‘mix_stream_new’:
fretwork/mixstream/MixStream.c:75:5: warning: ‘g_thread_init’ is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:261) [-Wdeprecated-declarations]

As we're using glib 2.34.3+, g_thread_init can be removed.
Eventually, we can add a condition on the version of glib.