tools headers UAPI: Sync linux/perf_event.h with the kernel sources

tools headers UAPI: Sync linux/perf_event.h with the kernel sources

To pick the trivial change in:

  ddecd22878601a60 ("perf: uapi: Document perf_event_attr::sig_data truncation on 32 bit architectures")

Just adds a comment.

This silences this perf build warning:

  Warning: Kernel ABI header at 'tools/include/uapi/linux/perf_event.h' differs from latest version at 'include/uapi/linux/perf_event.h'
  diff -u tools/include/uapi/linux/perf_event.h include/uapi/linux/perf_event.h

Cc: Marco Elver <elver@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/lkml/
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>