glsl: Scaffolding for ARB_shader_bit_encoding.

That adds support for activating the extension.  It doesn't actually
*do* anything yet, of course.

Signed-off-by: Olivier Galibert <galibert@pobox.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/glcpp/glcpp-parse.y

@@ -1141,6 +1141,9 @@ glcpp_parser_create (const struct gl_extensions *extensions, int api)
 
 	   if (extensions->OES_EGL_image_external)
 	      add_builtin_define(parser, "GL_OES_EGL_image_external", 1);
+
+	   if (extensions->ARB_shader_bit_encoding)
+	      add_builtin_define(parser, "GL_ARB_shader_bit_encoding", 1);
 	}
 
 	language_version = 110;