Remove using std::string from port.h

Change-Id: I7376f5e7eace22ec1fc05a61eaa858594f08682d

internal/ceres/residual_block_utils.cc

diff --git a/internal/ceres/residual_block_utils.cc b/internal/ceres/residual_block_utils.cc
index d2564a7..9bd8f6b 100644
--- a/internal/ceres/residual_block_utils.cc
+++ b/internal/ceres/residual_block_utils.cc
@@ -44,6 +44,8 @@
 namespace ceres {
 namespace internal {
 
+using std::string;
+
 void InvalidateEvaluation(const ResidualBlock& block,
                           double* cost,
                           double* residuals,