perf tools: Add overhead/overhead_children keys defaults via string

perf tools: Add overhead/overhead_children keys defaults via string

We currently set 'overhead' and 'overhead_children' as default sort keys
within perf_hpp__init function by directly adding into the sort list.

This patch adds 'overhead' and 'overhead_children' in text form into
sort_keys and let them be added by standard sort dimension interface.

We need to eliminate dirrect sort_list additions to be able to add
support for hists specific sort keys.

Signed-off-by: Jiri Olsa <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Noel Grandin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>