perf hists: Add more helper functions for the hierarchy mode

The hists__overhead_width() is to calculate width occupied by the
overhead (and others) columns before the sort columns.

The hist_entry__has_hiearchy_children() is to check whether an entry has
lower entries (children) in the hierarchy to be shown in the output.
This means the children should not be filtered out and above the percent
limit.

These two functions will be used to show information when all children
of an entry is omitted by the percent limit (or filter).

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1456488800-28124-1-git-send-email-namhyung@kernel.org


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