um: Use os_set_pdeathsig helper in winch thread/process

Since we have a helper now, let's switch to using it. It will make
the code slightly more consistent.

Signed-off-by: Tiwei Bie <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

arch/um/drivers/chan_user.c

···
 #include <signal.h>
 #include <termios.h>
 #include <sys/ioctl.h>
-#include <sys/prctl.h>
 #include "chan_user.h"
 #include <os.h>
 #include <um_malloc.h>
···
 	int count;
 	char c = 1;
 
-	prctl(PR_SET_PDEATHSIG, SIGKILL);
+	os_set_pdeathsig();
 
 	pty_fd = data->pty_fd;
 	pipe_fd = data->pipe_fd;