meson: Add macOS definitions for qmlgl plugin

Tested with Cerbero.
8 jobs for <span class="ref-name">fix-qt-ios-android-meson</span> in 14 minutes and 4 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #92203
gst indent

00:00:19

passed #92202
manifest

00:00:39

 
  Build
passed #92204
build fedora x86_64

00:02:10

 
  Test
passed #92205
check fedora

00:03:31

 
  Full Builds
passed #92206
build android arm64 api28

00:03:09

passed #92208
build cerbero cross win32

00:07:41

passed #92209
build cerbero cross win64

00:06:45

passed #92207
build cerbero fedora x86_64

00:07:10