When sequencer was moved from kernel, the DEFINES based on audaspace
were not moved along.
That means users had no way of creating new audio strips since
BKE_sequencer_add_sound_strip was ifdef'out
This is first step of refactoring task T77580.
Next step will be breaking up files into smaller ones.
Reviewed By: sergey, brecht
Differential Revision: https://developer.blender.org/D8492