Fix EEVEE: Shader error when using volume temperature or color attributes

This bug was unreported. This was triggering a linking error caused by
the vertex shader not having a local version of `attr_load_temperature_post`
and `attr_load_color_post`.

source/blender/draw/engines/eevee/shaders/volumetric_vert.glsl

@@ -76,4 +76,14 @@ vec3 coordinate_reflect(vec3 P, vec3 N)
 vec3 coordinate_incoming(vec3 P)
 {
   return vec3(0.0);
-}
+}
+
+float attr_load_temperature_post(float attr)
+{
+  return attr;
+}
+
+vec4 attr_load_color_post(vec4 attr)
+{
+  return attr;
+}