build: Only add libdav1d_nasm_objs if needed

Current versions of meson have a bug that causes the need to add the nasm generated objects to checkasm, even though this should already be covered by the extract_all_objects() for libdav1d. Meson versions >= 0.48.999 (that is, Meson 0.49 and development versions states of that on git) fixed this issue so now adding this is not longer needed. Adding it regardless would actually cause an error because of symbols being present twice.