mac80211: Remove duplicate linux/slab.h include from net/mac80211/scan.c

Commit 79f460ca add a duplicate
linux/slab.h include to net/mac80211/scan.c - remove it.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>