servo: Merge #8888 - stub out gluCheckExtension in embedding (from zmike:stubbed_my_skia); r=larsbergstrom

this is called from src/gpu/gl/unix/SkNativeGLContext_unix.cpp which
is not something that is used in servo

ref #8883

Source-Repo: https://github.com/servo/servo
Source-Revision: f5eec35cb47722f35cb1412eab2c096bb22fd6cc

servo/ports/cef/stubs.rs

@@ -51,3 +51,5 @@ stub!(cef_string_wide_to_utf16);
 stub!(cef_unregister_internal_web_plugin);
 stub!(cef_visit_web_plugin_info);
 
+//from skia
+stub!(gluCheckExtension);