conditionally include hnsw source (#2135)

This PR conditionally includes `hnsw` sources, to prevent build errors like those seen in cuGraph after #2022 was merged. See also: rapidsai/cugraph#4121, rapidsai/cugraph#4122

Authors:
   - Divye Gala (https://github.com/divyegala)

Approvers:
   - Corey J. Nolet (https://github.com/cjnolet)
   - Bradley Dice (https://github.com/bdice)
   - Robert Maynard (https://github.com/robertmaynard)

cpp/CMakeLists.txt

@@ -433,7 +433,7 @@ if(RAFT_COMPILE_LIBRARY)
     src/raft_runtime/neighbors/cagra_search.cu
     src/raft_runtime/neighbors/cagra_serialize.cu
     src/raft_runtime/neighbors/eps_neighborhood.cu
-    src/raft_runtime/neighbors/hnsw.cpp
+    $<$<BOOL:${BUILD_CAGRA_HNSWLIB}>:src/raft_runtime/neighbors/hnsw.cpp>
     src/raft_runtime/neighbors/ivf_flat_build.cu
     src/raft_runtime/neighbors/ivf_flat_search.cu
     src/raft_runtime/neighbors/ivf_flat_serialize.cu