The ES3.1 spec, and discussion on Khronos.org, confirm that dEQP is
correct in accepting mismatching centroid specifiers in shader
linkage. Mismatching flat/smooth is still a link error.

Fixes:
shaders.linkage.varying.rules.differing_interpolation_2

Change-Id: I3016f4147e7c1b16b02371ee95866c8daf826212
Reviewed-on: https://chromium-review.googlesource.com/251205Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>