Take argument by const& as the input range is const (#2558)

Found breaking CCCL ci

Authors:
  - Michael Schellenberger Costa (https://github.com/miscco)

Approvers:
  - Kyle Edwards (https://github.com/KyleFromNVIDIA)
  - Dante Gama Dessavre (https://github.com/dantegd)

URL: #2558

cpp/tests/neighbors/ball_cover.cu

@@ -78,7 +78,7 @@ RAFT_KERNEL count_discrepancies_kernel(value_idx* actual_idx,
 }
 
 struct is_nonzero {
-  __host__ __device__ bool operator()(uint32_t& i) { return i > 0; }
+  __host__ __device__ bool operator()(const uint32_t& i) { return i > 0; }
 };
 
 template <typename value_idx, typename value_t>