gdb: make target_ops::follow_fork return void

gdb: make target_ops::follow_fork return void

I noticed that all implementations return false, so
target_ops::follow_fork doesn't really need to return a value.  Change
it to return void.

gdb/ChangeLog:

	* target.h (struct target_ops) <follow_fork>: Return void.
	(target_follow_fork): Likewise.
	* target.c (default_follow_fork): Likewise.
	(target_follow_fork): Likewise.
	* infrun.c (follow_fork_inferior): Adjust.
	* fbsd-nat.h (class fbsd_nat_target) <follow_fork>: Return void.
	* fbsd-nat.c (fbsd_nat_target:::follow_fork): Likewise.
	* linux-nat.h (class linux_nat_target) <follow_fork>: Likewise.
	* linux-nat.c (linux_nat_target::follow_fork): Return void.
	* obsd-nat.h (class obsd_nat_target) <follow_fork>: Return void.
	* obsd-nat.c (obsd_nat_target::follow_fork): Likewise.
	* remote.c (class remote_target) <follow_fork>: Likewise.
	(remote_target::follow_fork): Likewise.
	* target-delegates.c: Re-generate.

Change-Id: If908c2f68b29fa275be2b0b9deb41e4c6a1b7879