Added missing SND_SEQ_EVENT_CLOCK event (MIDI Real Time Clock message) f...

...or timing ignoring.

The event ignoring mechanism is missing SND_SEQ_EVENT_CLOCK from ALSA, which is the event type associated with the MIDI message 0xF8 (Timing Clock). The used SND_SEQ_EVENT_TICK event is not an outside world MIDI message, but a special message generated by the ALSA synth.

I discovered this bug when working with a real Yamaha Clavinova Digital Piano, which was broadcasting 0xF8 messages and RtMidi wasn't properly ignoring them. This proposed patch correctly ignores that message, like the other drivers inside RtMidi.