perf thread: Adopt get_main_thread from db-export.c

Move the get_main_thread function from db-export.c to thread.c so that
it can be used elsewhere.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/1464051145-19968-2-git-send-email-andi@firstfloor.org


[ Removed leftover bits from db-export.h ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>