perf scripts python: exported-sql-viewer.py: Hide Call Graph option if no calls table

The Call Graph depends on the calls table which is optional when exporting
data, so hide the Call Graph option if there is no calls table.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>