perf db-export: Export main_thread in db_export__sample()

Export main_thread in db_export__sample() because it makes the code
easier to understand, and prepares db_export__thread() for further
simplification.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190710085810.1650-5-adrian.hunter@intel.com


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