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