Hi Adam,
It appears that you might have copied the files from CL31417988 on top of the 5.4 sources. This CL was submitted on Main around the 5.5 time and cannot be simply copied over. You must merge the changes for each file in the CL\commit.
We also suspect that your codebase might not have been properly changed when you updated from 5.3.x to 5.4.x. SkeletalMeshUpdate.cpp and SkeletalMeshUpdate.h have been around for a while now but they were relocated in version 5.4.
- Before 5.4: Engine\Source\Runtime\Engine\Private\Streaming\
- After 5.4 : Engine\Source\Runtime\Engine\Internal\Streaming\
Regards,
Martin