Fix CPU-only build for new FIL

[hcho3]

Command used to test building standalone FIL with -DCUML_ENABLE_GPU=OFF:

cmake -DCMAKE_INSTALL_PREFIX=$CONDA_PREFIX -DCMAKE_CUDA_ARCHITECTURES=75 \
  -DCMAKE_BUILD_TYPE=Release -DBUILD_CUML_C_LIBRARY=ON -DSINGLEGPU= \
  -DCUML_ALGORITHMS=fil -DBUILD_CUML_TESTS=OFF -DBUILD_CUML_MPI_COMMS=OFF \
  -DBUILD_CUML_MG_TESTS=OFF -DCUML_USE_TREELITE_STATIC=OFF -DNVTX=OFF \
  -DUSE_CCACHE=OFF -DDISABLE_DEPRECATION_WARNINGS=ON -DCMAKE_PREFIX_PATH=$CONDA_PREFIX  \
  -DCMAKE_MESSAGE_LOG_LEVEL=WARNING -DCUML_ENABLE_GPU=OFF -GNinja ..

This pull request fixes the CPU-only build with -DCUML_ENABLE_GPU=OFF.

[copy-pr-bot]

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[hcho3]

/ok to test

[ajschmidt8]

This PR has been marked untrusted due to the changes outlined in #5563.

It looks like the PR has unsigned commits.

You can rebase your branch to sign the unsigned commits to avoid having to solicit an /ok to test comment for every subsequent push.

The blog post below outlines how you can do this:

• https://duan.ca/2016/12/24/rebase-multiple-git-commits-with-signing/

[ajschmidt8]

/ok to test

[ajschmidt8]

btw, there is a useful FAQ about the /ok to test comment below:

• https://docs.gha-runners.nvidia.com/apps/copy-pr-bot/faqs/#why-didnt-my-ok-to-test-comment-work

[dantegd]

/merge