mesa: expose EXT_texture_compression_rgtc on GLES

The spec was modified to support GLES. Tested by piglit.

v2: rebase

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu> (v1)
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>

docs/relnotes/19.0.0.html

@@ -40,6 +40,7 @@ TBD.
 
 <ul>
 <li>GL_EXT_shader_implicit_conversions on all drivers (ES extension).</li>
+<li>GL_EXT_texture_compression_rgtc on all GL 3.0 drivers (ES extension).</li>
 <li>GL_EXT_texture_view on drivers supporting texture views (ES extension).</li>
 <li>GL_OES_texture_view on drivers supporting texture views (ES extension).</li>
 </ul>