mesa: Fix missing setting of shader->IsES.

mesa: Fix missing setting of shader->IsES.

I noticed this while trying to merge code with the builtin compiler, which
does set it.

Note that this causes two regressions in piglit in
default-precision-sampler.* which try to link without a vertex or fragment
shader, due to being run under the desktop glslparsertest binary (using
ARB_ES3_compatibility) that doesn't know about this requirement.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>