Fix kill of processes created by win32_create_inferior

handle_v_kill uses signal_pid because win32 doesn't support multi-process.

Without this gdb just refuses to kill the process:
(gdb) kill
Kill the program being debugged? (y or n) y
Sending packet: $vKill;a410#33...Packet received: E01
Packet vKill (kill) is supported
Can't kill process

gdbserver/ChangeLog:

2020-02-12  Hannes Domani  <ssbssa@yahoo.de>

	* win32-low.c (win32_create_inferior): Set signal_pid.