perf tools: Add stat round event synthesize function

Introduce the perf_event__synthesize_stat_round function to
synthesize a 'struct stat_round_event'.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Kan Liang <kan.liang@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1445784728-21732-19-git-send-email-jolsa@kernel.org


[ Renamed 'time' parameter to 'evtime' to fix build on older systems ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>