examples/gl/gtk: Fix build on macOS

The gdkquartz.h include pulls into Apple Obj-C frameworks so the compiler needs
to handle the gstgtkhelper library as such.

Fixes #518
6 jobs for gtkgl-example-meson-fix in 29 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #69906
gst indent

00:00:33

passed #69905
manifest

00:00:22

 
  Build
passed #69907
build fedora x86_64

00:04:45

 
  Test
passed #69908
check fedora

00:16:16

 
  Full Builds
passed #69909
build android arm64 api28

00:03:53

passed #69910
build cerbero fedora x86_64

00:07:46