Fix another missing declaration warning

Change-Id: I895e16f3e43792bd7f92560709f9ebf6374c4777

internal/ceres/dense_qr_test.cc

diff --git a/internal/ceres/dense_qr_test.cc b/internal/ceres/dense_qr_test.cc
index b33de35..ac1258d 100644
--- a/internal/ceres/dense_qr_test.cc
+++ b/internal/ceres/dense_qr_test.cc
@@ -47,10 +47,14 @@
 
 typedef DenseLinearAlgebraLibraryType Param;
 
+namespace {
+
 std::string ParamInfoToString(testing::TestParamInfo<Param> info) {
   return DenseLinearAlgebraLibraryTypeToString(info.param);
 }
 
+}  // namespace
+
 class DenseQRTest : public ::testing::TestWithParam<Param> {};
 
 TEST_P(DenseQRTest, FactorAndSolve) {