perf scripts python: exported-sql-viewer.py: Add tree level

As preparation for adding support for copying to clipboard, keep track of
what level each item is in tree items.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190503120828.25326-4-adrian.hunter@intel.com


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