ao_dsound.c depended on the same configure check as vo_directx.c, which
was removed in commit 0e2c48a3ce. This accidentally disabled
inclusion of ao_dsound.
Fix it by adding a new check. Also, move it below ao_portaudio on the
auto-select list, as ao_dsound is considered deprecated.
Unrelated to that, move ao_lavc below ao_null to prevent it from being
auto-selected.