Eliminate CUDA set but unused variable warning

This avoids warnings such as

    D:\a\ceres-solver\ceres-solver\internal\ceres\cuda_kernels_bsm_to_crs.cu.cc(143): warning #550-D: variable "row_nnz_e" was set but never used [D:\a\ceres-solver\ceres-solver\build_Release\internal\ceres\ceres_cuda_kernels.vcxproj]
          int row_nnz_e = 0;
              ^
                detected during instantiation of "void ceres::internal::RowBlockIdAndNNZ(int, int, int, const int *, const ceres::internal::Cell *, const ceres::internal::Block *, const ceres::internal::Block *, int *, int *, int *) [with partitioned=false]" at line 240

      Remark: The warnings can be suppressed with "-diag-suppress <warning-number>"

that is caused by compile-time conditional use of the variable.

Change-Id: I041b076a5208eeb662b8c7500d7b238965be833c