glsl: Check that interpolation qualifiers do not precede 'varying'

glsl: Check that interpolation qualifiers do not precede 'varying'

... and 'centroid varying'. The check is performed only in GLSL
versions >= 1.30.

From page 29 (page 35 of the PDF) of the GLSL 1.30 spec:
   "interpolation qualifiers may only precede the qualifiers in, centroid
    in, out, or centroid out in a declaration. They do not apply to the
    deprecated storage qualifiers varying or centroid varying."

Fixes Piglit test
spec/glsl-1.30/compiler/interpolation-qualifiers/smooth-varying-01.frag.