Make sure std::string constructor is available

.. because <stdexcept> only needs to have a forward declaration of
std::string. But that's not enough to invoke constructors of
std::exception (and derivatives) if we don't have a reference to an
std::string handy.

include/boost/container/throw_exception.hpp

@@ -24,6 +24,7 @@
 
 #ifndef BOOST_NO_EXCEPTIONS
    #include <stdexcept> //for std exception types
+   #include <string>    //for implicit std::string conversion
    #include <new>       //for std::bad_alloc
 #else
    #include <boost/assert.hpp>