[patch][gold] Fix symbol visibility in plugin objects

[Cary Coutant]

> The subject says it all :-)
>
> 2010-05-30  Rafael Espindola  <espindola@google.com>
>
>        * plugin.cc (Sized_pluginobj::::do_add_symbols): Correctly set the
>        visibility of symbols.

Ouch. Did I really write that? LGTM, and thanks!

-cary