openal: Fix AL header path for Mac OS X

OpenAL.framework doesn't provide 'alext.h'.
Thus, openal is enabled only if the dependency
is set by pkg-config.

fixes: #852
6 jobs for pr/openal-fix-on-macosx in 28 minutes and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #69429
gst indent

00:00:28

passed #69428
manifest

00:00:16

 
  Build
passed #69430
build fedora x86_64

00:03:25

 
  Test
passed #69431
check fedora

00:15:03

 
  Full Builds
passed #69432
build android arm64 api28

00:05:10

passed #69433
build cerbero fedora x86_64

00:09:12