perf tools: Move 'struct events_stats' and prototypes to separate header

This will allow us to untangle the header dependency a bit more, as some
places will not need event.h anymore.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-enqncj29ovzaat3cd9203rwl@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>