Include thrust/sort.h in ball_cover.cuh (#526)

New thrust release (11.6) has removed some unnecessary header includes. In order for raft to build, we need to include `thrust/sort.h` in `ball_cover.cuh`.

Authors:
  - Akif ÇÖRDÜK (https://github.com/akifcorduk)

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

URL: #526

cpp/include/raft/spatial/knn/detail/ball_cover.cuh

@@ -45,6 +45,7 @@
 #include <thrust/functional.h>
 #include <thrust/reduce.h>
 #include <thrust/sequence.h>
+#include <thrust/sort.h>
 
 namespace raft {
 namespace spatial {