perf hists: Introduce nr_header_lines into struct perf_hpp_list

Currently we support just single line headers, this is first step to
allow more.

Store the number of header lines in perf_hpp_list, which encompasses all
the display/sort entries and is thus suitable to hold this value.

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/1470583710-1649-2-git-send-email-jolsa@kernel.org


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