debugprintf example fails on macOS

[SRSaunders]

Adding tracking issue for defect:

[mvk-error] VK_ERROR_INITIALIZATION_FAILED: Shader library compile failed (Error code 3):
program_source:36:5: error: use of undeclared identifier 'debugPrintfEXT'
    debugPrintfEXT("Position = %v4f", pos);
    ^

Solution/workaround is a simple macOS-only change, as provided by #1124. Does not affect other platforms.