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 Name Job ID Coverage
  Preparation
passed gst indent #69625

00:00:30

passed manifest #69624

00:00:23

 
  Build
passed build fedora x86_64 #69626

00:02:00

 
  Test
passed check fedora #69627

00:14:22

 
  Full Builds
passed build android arm64 api28 #69628

00:04:04

passed build cerbero fedora x86_64 #69629

00:06:16