Use GLib compatible API to suppress warnings

    cut-run-context.c: In function 'cut_run_context_init':
    cut-run-context.c:965:5: warning: implicit declaration of function 'g_mutex_new' [-Wimplicit-function-declaration]
         priv->mutex = g_mutex_new();
         ^
    cut-run-context.c:965:17: warning: assignment makes pointer from integer without a cast [enabled by default]
         priv->mutex = g_mutex_new();
                     ^
    cut-run-context.c: In function 'dispose':
    cut-run-context.c:1014:9: warning: implicit declaration of function 'g_mutex_free' [-Wimplicit-function-declaration]
             g_mutex_free(priv->mutex);
             ^