"GLenum" needs to be prefixed to disambiguate between global GLenum
and sh::GLenum.

TEST=compile with MSVS 2015

Change-Id: I31a13c8d410bf32c4d5114c03f4b119aae256778
Reviewed-on: https://chromium-review.googlesource.com/746761Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>