examples/gl: Cocoa example Meson build definitions

tests/examples/gl/cocoa/meson.build

+
+if host_system == 'darwin'
+  executable('cocoa-videooverlay', 'cocoa-videooverlay.m',
+    objc_args : [gst_plugins_base_args],
+    include_directories: [configinc],
+    dependencies : [gstgl_dep,],
+    install: false)
+endif

tests/examples/gl/meson.build

-# FIXME: subdir('cocoa')
+subdir('cocoa')
 subdir('generic')
 # FIXME: gtk is temporarily disabled. 
 #        See. https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/issues/518