[patch] Fix mismatched #pragma GCC visibility

[Paolo Carlini]

>> Thanks for raising the issue. To me the above looks like a
>> straightforward typo, which we should fix as soon as possible (not
>> sure whether we could also diagnose it).
>>
>> Jason, Jakub, can you confirm?
> While we can change it, it is certainly valid as is, #pragma GCC visibility
> can happen in lots of places and just marks where the default visibility
> is adjusted.
Indeed, thanks for clarifying. In the specific case of libsupc++ I guess 
it's essentially a matter of style / consistency the exact line. We can 
schedule the change for Stage 1.
>    If clang errors on this, it is broken.
Ehm, agreed.

Paolo.