Dwarf4 clarified that only FORM_sec_offset can refer to location
lists, fix dwarf2_compute_location_attr accordingly.

This fixes the 'fixme:dwarf2_parse_udt_member: Found register, while
not expecting it' messages.
Signed-off-by: Eric Pouech <eric.pouech@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>