ACPI / NVS: Include appropriate header file in nvs.c

Include header file internal.h in nvs.c because functions
suspend_nvs_free(), suspend_nvs_alloc(), suspend_nvs_save() and
suspend_nvs_restore() have their prototype declaration in internal.h.

This eliminates the following warnings in nvs.c:
drivers/acpi/nvs.c:128:6: warning: no previous prototype for ‘suspend_nvs_free’ [-Wmissing-prototypes]
drivers/acpi/nvs.c:152:5: warning: no previous prototype for ‘suspend_nvs_alloc’ [-Wmissing-prototypes]
drivers/acpi/nvs.c:169:5: warning: no previous prototype for ‘suspend_nvs_save’ [-Wmissing-prototypes]
drivers/acpi/nvs.c:201:6: warning: no previous prototype for ‘suspend_nvs_restore’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>