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>

src/mesa/program/ir_to_mesa.cpp

@@ -3155,6 +3155,7 @@ _mesa_glsl_compile_shader(struct gl_context *ctx, struct gl_shader *shader)
    shader->CompileStatus = !state->error;
    shader->InfoLog = state->info_log;
    shader->Version = state->language_version;
+   shader->IsES = state->es_shader;
    memcpy(shader->builtins_to_link, state->builtins_to_link,
 	  sizeof(shader->builtins_to_link[0]) * state->num_builtins_to_link);
    shader->num_builtins_to_link = state->num_builtins_to_link;