ACPI: NFIT: Add declaration in a local header

The nfit_intel_shutdown_status() function has a __weak defintion
in nfit.c and an override in acpi_nfit_test.c for testing
purposes. This works without an extern declaration, but causes
a W=1 build warning:

drivers/acpi/nfit/core.c:1717:13: error: no previous prototype for 'nfit_intel_shutdown_status' [-Werror=missing-prototypes]

Add a declaration in a header that gets included from both
sides to shut up the warning and ensure that the prototypes
actually match.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>