Align gst_mixer_oss_set_volume prototype with _GstMixerClass definition.

Commit 76572502 added a gint num_channels argument to the _GstMixerClass set_volume prototype, but the oss mixer code was not updated accordingly.

I'm assuming this parameter is actually unused in the oss mixer, and only adding it to the function definition.

This is necessary since clang 16 (default compiler on recent FreeBSD head and future FreeBSD 14.0) is very strict.