ACPI: Rename configfs.c to acpi_configfs.c to prevent link error

If we compile ACPI configfs.c as module it will confuse the linker as it
hides symbols from the actual configfs:

Kernel: arch/x86/boot/bzImage is ready  (#1236)
  MODPOST 5739 modules
ERROR: "configfs_unregister_subsystem" [samples/configfs/configfs_sample.ko] undefined!
ERROR: "configfs_register_subsystem" [samples/configfs/configfs_sample.ko] undefined!
ERROR: "config_group_init" [samples/configfs/configfs_sample.ko] undefined!
ERROR: "config_item_init_type_name" [samples/configfs/configfs_sample.ko] undefined!
ERROR: "config_group_init_type_name" [samples/configfs/configfs_sample.ko] undefined!
ERROR: "configfs_undepend_item" [fs/ocfs2/cluster/ocfs2_nodemanager.ko] undefined!
...

Prevent these by renaming the file to acpi_configfs.c instead.

Reported-by: Scott Lawson <scott.lawson@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>