Fix for compiler warning on older gcc versions (4.1.2-42 and 4.4.6-4)

introduced by previous "bt -c" patch:

  kernel.c: In function ‘cmd_bt’:
  kernel.c:1981: warning: ‘cpus’ may be used uninitialized in this function

kernel.c

@@ -1989,6 +1989,7 @@ cmd_bt(void)
 	char arg_buf[BUFSIZE];
 
 	tc = NULL;
+	cpus = NULL;
 	subsequent = active = choose_cpu = 0;
 	hook.eip = hook.esp = 0;
 	refptr = 0;