perf stat: Move 'interval' into struct perf_stat_config

Moving 'interval' into struct perf_stat_config. The point is to
centralize the base stat config so it could be used localy together with
other stat routines in other parts of perf code.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
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/1437481927-29538-6-git-send-email-jolsa@kernel.org


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