gstaudiosrc/sink: Set audio ringbuffer thread priority

gstaudiosrc/sink: Set audio ringbuffer thread priority

On Windows, the ringbuffer thread function must have the "Pro Audio"
priority set, otherwise it sometimes doesn't get scheduled for
200-300ms, which will immediately cause an underrun unless you set
a very high latency-time and buffer-time.

This has no compile-time deps since it tries to load avrt.dll at
runtime to set the thread priority.