Explicitly import string into the Ceres namespace.

This is a workaround for anyone building Ceres in an environment
where there is a non-standard string implementation in the global
namespace. Due to the way the standard is written, a "using
namespace X" import is not high enough precedence to resolve a
naked reference to "string". Instead, by explicitly importing
string, the lookup becomes unambiguous.

Change-Id: I8d70463de01c482796c5bc09da05b37d21e7af96

include/ceres/internal/port.h

diff --git a/include/ceres/internal/port.h b/include/ceres/internal/port.h
index 9a3e5cc..a9fe247 100644
--- a/include/ceres/internal/port.h
+++ b/include/ceres/internal/port.h
@@ -31,6 +31,8 @@
 #ifndef CERES_PUBLIC_INTERNAL_PORT_H_
 #define CERES_PUBLIC_INTERNAL_PORT_H_
 
+#include <string>
+
 namespace ceres {
 
 // It is unfortunate that this import of the entire standard namespace is
@@ -39,6 +41,10 @@
 // things outside of the Ceres optimization package.
 using namespace std;
 
+// This is necessary to properly handle the case that there is a different
+// "string" implementation in the global namespace.
+using std::string;
+
 }  // namespace ceres
 
 #endif  // CERES_PUBLIC_INTERNAL_PORT_H_