Abstraction layer: Change setp to this->setp

- Upgrading gcc to 4.7 leads to the following error:
../src/dbal/OutputStreamBufferBase_impl.hpp:75:9: error: 'setp' was
not declared in this scope, and no declarations were found by
argument-dependent lookup at the point of instantiation [-fpermissive]
../src/dbal/OutputStreamBufferBase_impl.hpp:75:9: note: use 'this->setp' instead

- Making that change fixes the problem