Add VINCIA Userhook to CMakeLists in r21

Merged Michael James Fenton requested to merge mfenton/athena-2:mf_vincia_cmake into 21.6

The main31Vincia.cxx userhook was recently added to r22 and cherry picked to r21. Unfortunately, the r22 CMakeLists uses a wildcard to pick up the source files, while in r21 they are listed explicitly, and thus the new hook was not compiled in the most recent release. This MR corrects this mistake.

Once this is merged it would be great if a new release could be compiled in order to make a sample request.

Cheers, Mike

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