Fix missing declaration warning

Change-Id: I690b81af11bb98930d0a0d1c8d5d4d6d75f61bd3

internal/ceres/dense_cholesky_test.cc

diff --git a/internal/ceres/dense_cholesky_test.cc b/internal/ceres/dense_cholesky_test.cc
index 8d14f66..eb1c336 100644
--- a/internal/ceres/dense_cholesky_test.cc
+++ b/internal/ceres/dense_cholesky_test.cc
@@ -47,9 +47,12 @@
 
 typedef DenseLinearAlgebraLibraryType Param;
 
+namespace {
+
 std::string ParamInfoToString(testing::TestParamInfo<Param> info) {
   return DenseLinearAlgebraLibraryTypeToString(info.param);
 }
+}  // namespace
 
 class DenseCholeskyTest : public ::testing::TestWithParam<Param> {};