glsl: double-precision values don't support interpolation

glsl: double-precision values don't support interpolation

ARB_gpu_shader_fp64 spec says:

  "This extension does not support interpolation of double-precision
  values; doubles used as fragment shader inputs must be qualified as
  "flat"."

Fixes the regressions added by commit 781d278:

arb_gpu_shader_fp64-double-gettransformfeedbackvarying
arb_gpu_shader_fp64-tf-interleaved
arb_gpu_shader_fp64-tf-interleaved-aligned
arb_gpu_shader_fp64-tf-separate

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93878
Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>