tools headers UAPI: Update tools's copy of linux/perf_event.h

To get the changes in the commit:

  "perf: Add perf_event_mmap_page::cap_user_time_short ABI"

This update is a prerequisite to add support for short clock counters
related ABI extension.

Signed-off-by: Leo Yan <leo.yan@linaro.org>
Link: https://lore.kernel.org/r/20200716051130.4359-8-leo.yan@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>