ACPI / configfs: Mark local functions static

ACPI / configfs: Mark local functions static

There is no need to have non-static local functions. otherwise
compiler is not happy:

  CC [M]  drivers/acpi/acpi_configfs.o
drivers/acpi/acpi_configfs.c:105:9: warning: no previous prototype for ‘acpi_table_signature_show’ [-Wmissing-prototypes]
 ssize_t acpi_table_signature_show(struct config_item *cfg, char *str)
         ^~~~~~~~~~~~~~~~~~~~~~~~~
drivers/acpi/acpi_configfs.c:115:9: warning: no previous prototype for ‘acpi_table_length_show’ [-Wmissing-prototypes]
 ssize_t acpi_table_length_show(struct config_item *cfg, char *str)
         ^~~~~~~~~~~~~~~~~~~~~~
...

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>