Fix compiler warnings in PulseAudio backend:

backends/mixer_pulse.cpp: In member function 'bool Mixer_PULSE::addDevice(devinfo&, bool)':
backends/mixer_pulse.cpp:912: warning: this 'if' clause does not guard...
backends/mixer_pulse.cpp:915: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the 'if'
backends/mixer_pulse.cpp: In member function 'virtual bool Mixer_PULSE::moveStream(const QString&, const QString&)':
backends/mixer_pulse.cpp:1407: warning: enum constant in boolean context