LibCore: Don't wrap `<mach/mach.h>` in extern "C" on Darwin

This causes issues when loading the file into a clang module, such as
when using Core::MachPort from Objective-C or Swift.

Userland/Libraries/LibCore/MachPort.h

@@ -15,9 +15,13 @@
 #include <AK/Error.h>
 #include <AK/Noncopyable.h>
 
+#if defined(AK_OS_MACOS) || defined(AK_OS_IOS)
+#    include <mach/mach.h>
+#else
 extern "C" {
-#include <mach/mach.h>
+#    include <mach/mach.h>
 }
+#endif
 
 namespace Core {