Fix double definition error in ANN refinement header (#1067)

Authors:
  - Tamas Bela Feher (https://github.com/tfeher)

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

URL: #1067

cpp/include/raft/neighbors/detail/refine.cuh

@@ -141,7 +141,7 @@ typedef struct {
   float distance;
 } struct_for_refinement;
 
-int _postprocessing_qsort_compare(const void* v1, const void* v2)
+inline int _postprocessing_qsort_compare(const void* v1, const void* v2)
 {
   // sort in ascending order
   if (((struct_for_refinement*)v1)->distance > ((struct_for_refinement*)v2)->distance) {