ARM: S3C64XX: Add linux/export.h to dev-spi.c

This is required for the EXPORT_SYMBOL()s the code uses, previously the
header was being included implicitly by another header.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>