Now that a thread's current comm is exported, it shows up in the call
graph and call tree even if it has no calls. That can happen because the
calls are recorded against the main thread's initial comm.

Add a table column to make it easy for the exported-sql-viewer.py script
to select only comms with calls.

Committer notes:

Running the export-to-sqlite.py worked without warnings and using the
exported-sql-viewer.py worked as before.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190710085810.1650-14-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>