mac: Fix building on macOS 14.

[harukasan]

When I built on macOS 14, the following error occurred. This error is a recurrence of the issue fixed in #280.

/Users/harukasan/works/src/github.com/andrewrk/libsoundio/src/coreaudio.c:26:9: error: 'kAudioObjectPropertyElementMaster' is deprecated: first deprecated in macOS 12.0 [-Werror,-Wdeprecated-declarations]
        kAudioObjectPropertyElementMain
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        kAudioObjectPropertyElementMain
/Users/harukasan/works/src/github.com/andrewrk/libsoundio/src/coreaudio.c:16:41: note: expanded from macro 'kAudioObjectPropertyElementMain'
#define kAudioObjectPropertyElementMain kAudioObjectPropertyElementMaster
                                        ^

MAC_OS_VERSION_12_0 is not defined in macOS 14. Therefore, I fixed the version check to use MAC_OS_X_VERSION_MIN_REQUIRED.

I only have a macOS 14 environment, so I cannot test on earlier versions.
Please verify that there are no issues in older environments before merging.