Small follow-up to specializations cleanup (rapidsai#1332)

Just making sure the knn bench will properly compile even if only the distance library is compiled.

Authors:
  - Corey J. Nolet (https://github.com/cjnolet)

Approvers:
  - Ben Frederickson (https://github.com/benfred)

URL: rapidsai#1332

cpp/bench/neighbors/knn.cuh

@@ -26,13 +26,10 @@
 
 #if defined RAFT_DISTANCE_COMPILED
 #include <raft/neighbors/specializations.cuh>
-
-// TODO: Legacy. Remove when FAISS is removed
-#include <raft/spatial/knn/specializations.cuh>
 #endif
 
 #if defined RAFT_NN_COMPILED
-#include <raft/neighbors/specializations.cuh>
+// TODO: Legacy. Remove when FAISS is removed
 #include <raft/spatial/knn/specializations.cuh>
 #endif