ACPI: BGRT: use static for BGRT_SHOW kobj_attribute defines

ACPI: BGRT: use static for BGRT_SHOW kobj_attribute defines

Smatch reports this repesentative issue:

bgrt.c:26:1: warning: symbol 'bgrt_attr_version' was not declared. Should it be static?

Similar for *status,type,xoffset,yoffset

These variables are defined with the BGRT_SHOW macro.

For the definition of bgrt_attr_##_name,
the storage-class specifier should be static.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>