perf scripts python: export-to-sqlite.py: Fix use of parent_id in calls_view

perf scripts python: export-to-sqlite.py: Fix use of parent_id in calls_view

Fix following error using calls_view:

 Query failed: ambiguous column name: parent_id Unable to execute statement

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Fixes: bd296d2d2815 ("perf scripts python: export-to-sqlite.py: Export calls parent_id")
Link: http://lkml.kernel.org/r/20190409062557.26138-1-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>