build: Only add libdav1d_nasm_objs if needed
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.

videolan/dav1d