Remove using std::string from port.h

Change-Id: I7376f5e7eace22ec1fc05a61eaa858594f08682d
diff --git a/internal/ceres/suitesparse.cc b/internal/ceres/suitesparse.cc
index d8b1a49..c49af3c 100644
--- a/internal/ceres/suitesparse.cc
+++ b/internal/ceres/suitesparse.cc
@@ -44,6 +44,7 @@
 namespace ceres {
 namespace internal {
 
+using std::string;
 using std::vector;
 
 SuiteSparse::SuiteSparse() {