glsl: In later GLSL versions, sequence operator is cannot be a constant expression

glsl: In later GLSL versions, sequence operator is cannot be a constant expression

Fixes:
    ES3-CTS.shaders.negative.constant_sequence

    spec/glsl-es-3.00/compiler/global-initializer/from-sequence.vert
    spec/glsl-es-3.00/compiler/global-initializer/from-sequence.frag

v2: Fix a couple copy-and-paste mistake in the spec quotations.
Suggested by Matt.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: "10.6 11.0" <mesa-stable@lists.freedesktop.org>