trace: pass -X to less

Option -F should always be used with -X. Without this flag if output
is smaller than the screen then no output will be shown.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>