virtual_bt_speaker(8): Do not build if WITHOUT_BLUETOOTH is set

virtual_bt_speaker(8) includes bluetooth.h, which won't exist if
WITHOUT_BLUETOOTH is set, which will result in a build failure.

Reported by: peterj
Fixes: 9cab9fde5eda ("virtual_oss: Port to base")
Sponsored by: The FreeBSD Foundation
MFC after: 3 days