sk-packet.c: fixed a format string warning on ARM

Use a PRI* format specifier to convert an integer of known size
to a string.

Cc: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>