ACPI: Do not export functions that are only used in osl.c

The functions acpi_os_map_generic_address() and
acpi_os_unmap_generic_address() are only used in drivers/acpi/osl.c,
so make them static and remove the extern definitions of them from
include/linux/acpi_io.h.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>