perf: Move perf_event_fasync() to perf_event.h

This will allow it to be called from perf_output_wakeup().

Signed-off-by: Kyle Huey <khuey@kylehuey.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240413141618.4160-2-khuey@kylehuey.com