gdbserver: linux-low: make linux_process_target::filter_event return void

Same as the previous patch, but for GDBserver.  The return value of this
method is never used, change it to return void.

gdbserver/ChangeLog:

	* linux-low.cc (linux_process_target::filter_event): Return
	void.
	* linux-low.h (class linux_process_target) <filter_event>:
	Return void.

Change-Id: I79e5dc04d9b21b9f01c6d675fa463d1b1a703b3a