Eevee: Fix broken shader if only using an Ambient Occlusion node

This quick fix does generate some invalid uniform. Will fix that later on.

source/blender/nodes/shader/nodes/node_shader_ambient_occlusion.c

@@ -47,6 +47,8 @@ static int node_shader_gpu_ambient_occlusion(GPUMaterial *mat, bNode *node, bNod
 	if (!in[2].link)
 		GPU_link(mat, "world_normals_get", &in[2].link);
 
+	GPU_material_flag_set(mat, GPU_MATFLAG_DIFFUSE);
+
 	return GPU_stack_link(mat, node, "node_ambient_occlusion", in, out);
 }