build: Split out iSAC VAD sources into a separate dependency

Avoid having to link webrtc-audio-processing with webrtc-audio-coding,
and makes the required symbols directly available.

Part-of: <pulseaudio/webrtc-audio-processing!22>
4 jobs for isac-dep-fix in 1 minute and 12 seconds
Status Job ID Name Coverage
  Container
passed #11062446
aarch64
build-container-aarch64

00:00:24

passed #11062445
build-container-x86_64

00:00:19

 
  Build
passed #11062448
aarch64
build-aarch64

00:00:46

passed #11062447
build-x86_64

00:00:30