PATCH: reinstate `using' statements in backward header

[Benjamin Kosnik]

In thanks.

> g++.old-deja/g++.robertl/eb39.C fails without this patch which
> reinstates the `using' statements lost in a recent reorganization.
> 
> 2000-11-22  Loren J. Rittle  <ljrittle@acm.org>
> 
> 	* include/backward/strstream.h: Reinstate the `using'
> 	statements.