missing include guards on {bits,tr1}/shared_ptr.h

[Paolo Carlini]

Hi Jon,
> This is not technically a regression, because those headers didn't
> exist in GCC 4.3 and are not meant to be included by users anyway.
> Should a fix wait for 4.4.1, or can it go in now?
>   
Can go in now, definitely.

Many thanks,
Paolo.