perf hists: Add support for header span

Add span argument for header callback function.

The handling of this argument is completely in the hands of the
callback. The only thing the caller ensures is it's zeroed on the
beginning.

Omitting span skipping in hierarchy headers and gtk code.

The c2c code use this to span header lines based on the entries span
configuration.

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/1470583710-1649-6-git-send-email-jolsa@kernel.org


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