audiofx: add stereo element which was moved from -bad to build

Fixes gstreamer/gst-plugins-bad#457
6 jobs for audiofx-move in 23 minutes and 10 seconds
Status Job ID Name Coverage
  Preparation
passed #69625
gst indent

00:00:30

passed #69624
manifest

00:00:23

 
  Build
passed #69626
build fedora x86_64

00:02:00

 
  Test
passed #69627
check fedora

00:14:22

 
  Full Builds
passed #69628
build android arm64 api28

00:04:04

passed #69629
build cerbero fedora x86_64

00:06:16