Use GLib compatible API to suppress warnings

    cut-xml-stream.c: In function 'init':
    cut-xml-stream.c:138:5: warning: 'g_mutex_new' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:272) [-Wdeprecated-declarations]
         stream->mutex = g_mutex_new();
         ^
    cut-xml-stream.c: In function 'dispose':
    cut-xml-stream.c:222:9: warning: 'g_mutex_free' is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:274) [-Wdeprecated-declarations]
             g_mutex_free(stream->mutex);
             ^