Use benchmark version 1.6 compatible syntax.

This is needed to make the dense_linear_solver_benchmark.cc
compile with the currently stable versions of the benchmark
library available on various linux distributions.

Change-Id: I1e391d5c2d16250d213bcfa3d50f9560aad9a363

internal/ceres/dense_linear_solver_benchmark.cc

diff --git a/internal/ceres/dense_linear_solver_benchmark.cc b/internal/ceres/dense_linear_solver_benchmark.cc
index e949eb8..fa0d034 100644
--- a/internal/ceres/dense_linear_solver_benchmark.cc
+++ b/internal/ceres/dense_linear_solver_benchmark.cc
@@ -87,13 +87,15 @@
   b->Args({800, 25});
 }
 
-BENCHMARK(BM_DenseSolver<ceres::EIGEN, ceres::DENSE_QR>)->Apply(MatrixSizes);
-BENCHMARK(BM_DenseSolver<ceres::EIGEN, ceres::DENSE_NORMAL_CHOLESKY>)
+BENCHMARK_TEMPLATE2(BM_DenseSolver, ceres::EIGEN, ceres::DENSE_QR)
+    ->Apply(MatrixSizes);
+BENCHMARK_TEMPLATE2(BM_DenseSolver, ceres::EIGEN, ceres::DENSE_NORMAL_CHOLESKY)
     ->Apply(MatrixSizes);
 
 #ifndef CERES_NO_LAPACK
-BENCHMARK(BM_DenseSolver<ceres::LAPACK, ceres::DENSE_QR>)->Apply(MatrixSizes);
-BENCHMARK(BM_DenseSolver<ceres::LAPACK, ceres::DENSE_NORMAL_CHOLESKY>)
+BENCHMARK_TEMPLATE2(BM_DenseSolver, ceres::LAPACK, ceres::DENSE_QR)
+    ->Apply(MatrixSizes);
+BENCHMARK_TEMPLATE2(BM_DenseSolver, ceres::LAPACK, ceres::DENSE_NORMAL_CHOLESKY)
     ->Apply(MatrixSizes);
 #endif