perf/ring_buffer: Trigger IO signals for watermark_wakeup

perf_output_wakeup() already marks the perf event fd available for polling.
Trigger IO signals with FASYNC too.

Signed-off-by: Kyle Huey <khuey@kylehuey.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240413141618.4160-3-khuey@kylehuey.com