Load functions from ARB_uniform_buffer_object

Load functions from ARB_uniform_buffer_object

While this extension is part of OpenGL 3.1 and above, it is not part of
OpenGL 3.0.

As we need to run the tests for OpenGL 3.0 too, and we need to use some
functions exposed by this extension, let's load them from the extension
itself, instead of relying in the core functions.

Affects:
* KHR-GL30.transform_feedback.draw_xfb_instanced_test
* KHR-GL30.transform_feedback.draw_xfb_stream_instanced_test

Components: OpenGL

VK-GL-CTS issue: 819

Change-Id: I2cd1652b4c2d3b55e009c44b85b451d207387b96