Use "bool" in fetch_inferior_event

A while back I noticed that fetch_inferior_event used "int" for
should_stop, whereas it can be bool.  The method it is assigned from:

	      should_stop = thread_fsm->should_stop (thr);

... already returns bool.

Tested by rebuilding.

gdb/ChangeLog
2020-11-14  Tom Tromey  <tom@tromey.com>

	* infrun.c (fetch_inferior_event): Use "bool" for should_stop.