[GTK] Include the right GL header for GLES2
https://bugs.webkit.org/show_bug.cgi?id=113034

Reviewed by Martin Robinson.

Include GLES2/gl2.h when using GLES2.

Source/WebKit/gtk:

* WebCoreSupport/AcceleratedCompositingContextGL.cpp:

Source/WebKit2:

* WebProcess/WebPage/gtk/LayerTreeHostGtk.cpp:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146637 268f45cc-cd09-0410-ab3c-d52691b4dbfc