fixed compilation for PION_DISABLE_LOGGING and PION_USE_OSTREAM_LOGGING

Pion failed to compile, if cmake can't find log4cplus library.
If PION_DISABLE_LOGGING not defined logger used PION_USE_OSTREAM_LOGGING, but compiler can't find pion::logger::shutdown() method.
Same issue is for PION_DISABLE_LOGGING defined.