[REVIEW] Include knn.cuh in knn.cu benchmark source for finding brute_force_knn

[teju85]

Current issue is that if you try to build only the benchmark code by doing ./build.sh bench, the file cpp/bench/spatial/knn.cu fails to compile with the following error:

...
./cpp/bench/spatial/knn.cu(177): error: namespace "raft::spatial::knn" has no member "brute_force_knn"
          detected during:
            instantiation of "void raft::bench::spatial::knn<ValT, IdxT, ImplT>::run_benchmark(benchmark::State &) [with ValT=float, IdxT=int64_t, ImplT=raft::bench::spa$ial::brute_force_knn<float, int64_t>]"
...

The issue is that this file doesn't include knn.cuh (Courtesy: @achirkin). This PR fixes the error.

[cjnolet]

rerun tests

[cjnolet]

rerun tests

[cjnolet]

@teju85 this is weird and I'm not sure why CI keeps timing out on this PR. I thought there was somehting wrong w/ CI at first but it doesn't seem to time out like this on the other PRs. maybe there's another template we need to consider instantiating here?

[cjnolet]

@teju85 can you try also including raft/distance/specializations.cuh inside raft/spatial/knn/specializations.cuh? I think the knn specializations, especially the fused l2 knn, implicitly assume the needed distance specializations have been compiled already.

[cjnolet]

@gpucibot merge