adding 'using std::flush' to backward/iostream.h

[Gabriel Dos Reis]

Phil Edwards <pedwards@disaster.jaj.com> writes:

| > That must have been a mistake, yes it is OK to have a 
| > 
| >         using std:: <manipulator>;
| 
| We just got a PR that other things like std::hex are not made available
| through the backward headers.  Should they be added to iostream.h or
| iomanip.h?  My inclination is the latter.

Agreed.

-- Gaby
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