nir/lower_uniforms_to_ubo: set explicit_binding on uniform_0

this variable is always bound to buffer index 0, so the binding info
here is actually useful

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7935>

src/compiler/nir/nir_lower_uniforms_to_ubo.c

@@ -144,6 +144,7 @@ nir_lower_uniforms_to_ubo(nir_shader *shader, int multiplier)
          nir_variable *ubo = nir_variable_create(shader, nir_var_mem_ubo, type,
                                                  "uniform_0");
          ubo->data.binding = 0;
+         ubo->data.explicit_binding = 1;
 
          struct glsl_struct_field field = {
             .type = type,