Remove anonymous namespace from null_mask.cuh (#8786)

Removed a non-compliant anonymous namespace from `null_mask.cuh`.  The contents has been left in the `cudf::detail` namespace.

Resolves #8781

Authors:
  - https://github.com/nvdbaranec

Approvers:
  - Karthikeyan (https://github.com/karthikeyann)
  - MithunR (https://github.com/mythrocks)

URL: #8786

cpp/include/cudf/detail/null_mask.cuh

@@ -142,8 +142,6 @@ void inplace_bitmask_binop(
   stream.synchronize();
 }
 
-namespace {
-
 /**
  * For each range `[first_bit_indices[i], last_bit_indices[i])`
  * (where 0 <= i < `num_ranges`), count the number of bits set outside the range
@@ -248,8 +246,6 @@ struct word_num_set_bits_functor {
   bitmask_type const* bitmask;
 };
 
-}  // anonymous namespace
-
 /**
  * @brief Given a bitmask, counts the number of set (1) bits in every range
  * `[indices_begin[2*i], indices_begin[(2*i)+1])` (where 0 <= i < std::distance(indices_begin,