Remove superfluous NOTIFY_COOKIE_LEN define

NOTIFY_COOKIE_LEN is defined in mqueue.h as well as mqueue.c
This patch removes redundant definition from mqueue.c

Signed-off-by: Michal Wronski <Michal.Wronski@motorola.com>
Signed-Off-By: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>