USkeletalMesh::Serialize should preload dependent skeleton

In the skeletal mesh serialize method, we rebuild the ref skeleton for the mesh via this call:

GetRefSkeleton().RebuildRefSkeleton(GetSkeleton(), bRebuildNameMap);

So there's a dependency on the skeleton asset. But since serialization order is not guaranteed, we can potentially end up in a situation where the skeleton hasn't been deserialized yet.

This issue seems to be project specific so is very difficult to repro. There's more info on the linked UDN.