HID: Export hid_field_extract()

Rename the function extract() to hid_field_extract(), make it external linkage
to allow the use from other modules.

Suggested-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Goffredo Baroncelli <kreijack@inwind.it>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>