ACPI / OSL: Mark the function acpi_table_checksum() as static

Marks the function acpi_table_checksum() as static in osl.c
because it is not used outside this file.

This eliminates the following warning in osl.c:
drivers/acpi/osl.c:547:11: warning: no previous prototype for
‘acpi_table_checksum’ [-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>