Add required dependencies of cocoa GL example for linking.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/782>

tests/examples/gl/cocoa/meson.build

@@ -1,8 +1,9 @@
-
 if host_system == 'darwin'
+  appkit_dep = dependency('appleframeworks', modules : ['AppKit'], required : true)
+
   executable('cocoa-videooverlay', 'cocoa-videooverlay.m',
     objc_args : [gst_plugins_base_args],
     include_directories: [configinc],
-    dependencies : [gstgl_dep,],
+    dependencies : [gstgl_dep, corefoundation_dep, appkit_dep],
     install: false)
 endif