Fix SPI build errors in exports.c

The spi.h include is no longer included in exports.h, so must be included
here.

This fixes these errors:

In file included from exports.c:41:0:
/home/sjg/trunk/src/third_party/u-boot/files/include/_exports.h: In function 'jumptable_init':
/home/sjg/trunk/src/third_party/u-boot/files/include/_exports.h:27:1: error: 'spi_init' undeclared (first use in this function)
/home/sjg/trunk/src/third_party/u-boot/files/include/_exports.h:27:1: note: each undeclared identifier is reported only once for each function it appears in
/home/sjg/trunk/src/third_party/u-boot/files/include/_exports.h:28:1: error: 'spi_setup_slave' undeclared (first use in this function)
/home/sjg/trunk/src/third_party/u-boot/files/include/_exports.h:29:1: error: 'spi_free_slave' undeclared (first use in this function)
/home/sjg/trunk/src/third_party/u-boot/files/include/_exports.h:30:1: error: 'spi_claim_bus' undeclared (first use in this function)
/home/sjg/trunk/src/third_party/u-boot/files/include/_exports.h:31:1: error: 'spi_release_bus' undeclared (first use in this function)
/home/sjg/trunk/src/third_party/u-boot/files/include/_exports.h:32:1: error: 'spi_xfer' undeclared (first use in this function)

Signed-off-by: Simon Glass <sjg@chromium.org>