MixStream: g_mutex_new is deprecated

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This warning was raised when compiling the MixStream extension:

fretwork/mixstream/MixStream.c: In function ‘mix_stream_new’:
fretwork/mixstream/MixStream.c:119:3: warning: ‘g_mutex_new’ is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:273) [-Wdeprecated-declarations]

As we're using glib 2.34.3+, g_mutex_new can be removed.
See https://developer.gnome.org/glib/stable/glib-Threads.html#GMutex for how it should be.
Eventually, we can add a condition on the version of glib.