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 <[e-mail]>
Reviewed-by: Dave Jiang <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/acpi/nfit/nfit.h

···
 void acpi_nfit_desc_init(struct acpi_nfit_desc *acpi_desc, struct device *dev);
 bool intel_fwa_supported(struct nvdimm_bus *nvdimm_bus);
 extern struct device_attribute dev_attr_firmware_activate_noidle;
+void nfit_intel_shutdown_status(struct nfit_mem *nfit_mem);
+
 #endif /* __NFIT_H__ */