Remove using std::string from port.h

Change-Id: I7376f5e7eace22ec1fc05a61eaa858594f08682d
diff --git a/internal/ceres/polynomial.cc b/internal/ceres/polynomial.cc
index 848fbee..361ab68 100644
--- a/internal/ceres/polynomial.cc
+++ b/internal/ceres/polynomial.cc
@@ -43,6 +43,7 @@
 namespace ceres {
 namespace internal {
 
+using std::string;
 using std::vector;
 
 namespace {