mtd: nand: Include linux/mtd/partitions.h in nand_base.h

This patch removes this compilation warning when CONFIG_MTD_PARTITIONS is
defined:

nand_base.c: In function 'nand_release':
nand_base.c:2922: warning: implicit declaration of function 'del_mtd_partitions'

Signed-off-by: Stefan Roese <sr@denx.de>