ACPICA: Utilities: Cleanup declarations of the acpi_gbl_debug_file global.

This global is acting as an OSL global variable, implemented in the
oswinxf.c and osunixxf.c.

This patch cleans up the definition of this variable so that new utilities
do not need to define it in order to link.

Linux kernel behaviour is not affected as the changes only applies to the
ACPICA userspace utilities which are not shipped in the kernel currently.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>