ao_coreaudio: fix compilation on OS X 10.7

Reverts one of the changes from 18777ecf. `kAudioObjectPropertyScopeOutput`
was introduced in the 10.8 SDK while `kAudioDevicePropertyScopeOutput` was
moved to `AudioHardwareDeprecated.h`. Since the deprecation is silent for now
(no warnings), just use the old constant.

Either way, they both evaluate to 'outp', and in the 10.8 SDK the deprecated
constant is defined in terms of the non-deprecated one.

Fixes #155