[U-Boot] Build breakage due to "Standalone Apps: Standalone apps should only need exports.h"

[Moffett, Kyle D]

Hi,

I was just rebasing and tweaking my board-support patch to send out
again and I noticed that the latest U-Boot master branch does not
build when the config option "CONFIG_CMD_SPI" is enabled:
  In file included from exports.c:41
  [...]/include/_exports.h: In function 'jumptable_init':
  [...]/include/_exports.h:27: error: 'spi_init' undeclared (first use in this function)
  [...]/include/_exports.h:27: error: (Each undeclared identifier is reported only once
  [...]/include/_exports.h:27: error: for each function it appears in.)
  [...]/include/_exports.h:28: error: 'spi_setup_slave' undeclared (first use in this function)
  [...]/include/_exports.h:29: error: 'spi_free_slave' undeclared (first use in this function)
  [...]/include/_exports.h:30: error: 'spi_claim_bus' undeclared (first use in this function)
  [...]/include/_exports.h:31: error: 'spi_release_bus' undeclared (first use in this function)
  [...]/include/_exports.h:32: error: 'spi_xfer' undeclared (first use in this function)