Use kIOMasterPortDefault if compiled with SDK<12

Compiling against Apple SDK older than 12.0 ends with errors:

error: use of undeclared identifier 'kIOMainPortDefault'

That is because `kIOMainPortDefault` was introduced in SDK 12.0
(Monterey) and before that the `kIOMasterPortDefault` should be used.

`kIOMasterPortDefault` was deprecated and was removed in SDK 12.0

References:
- https://developer.apple.com/documentation/iokit/kiomainportdefault
- https://developer.apple.com/documentation/iokit/kiomasterportdefault

c_src/mac/keyio_mac.hpp

@@ -6,6 +6,12 @@
 #include <map>
 #include <iostream>
 #include <mach/mach_error.h>
+#include <AvailabilityMacros.h>
+
+/* The name was changed from "Master" to "Main" in Apple SDK 12.0 (Monterey) */
+#if !defined (MAC_OS_X_VERSION_12_0) || (MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_12_0)
+    #define kIOMainPortDefault kIOMasterPortDefault
+#endif
 
 int init_sink(void);
 int exit_sink(void);