include strstream instead of strstream.h

[Benjamin Kosnik]

> ==========================================================================
> In file included from /usr/src/redhat/BUILD/gcc/obj-i386-linux/i386-pc-linux-gnu/libstdc++-v3/include/backward/strstream:51,
>                  from /usr/src/redhat/BUILD/gcc/obj-i386-linux/i386-pc-linux-gnu/libstdc++-v3/include/backward/strstream.h:33,
>                  from ../../../../libstdc++-v3/src/strstream.cc:50:
> /usr/src/redhat/BUILD/gcc/obj-i386-linux/i386-pc-linux-gnu/libstdc++-v3/include/backward/backward_warning.h:32:2: warning: #warning This file includes at least one deprecated or antiquated header. Please use the <X> header instead of <X.h> header for 'C++' includes and use the <cX> header instead of <X.h> header for 'C' includes. To disable this warning use -Wno-deprecated
> ==========================================================================

I'm not quite sure why this needs fixing. It's using deprecated bits, 
thus the error. What's the problem? 

> -#include <strstream.h>
> +#include <strstream>

... both of these headers are deprecated, by <sstream>. If this really 
bugs you for some reason, just set CXXFLAGS to use -Wno-deprecated in 
your build tree.

-benjamin