Remove unused include of Availability.h

http://trac.macports.org/ticket/46535

--- src/ld/parsers/libunwind/AddressSpace.hpp
+++ src/ld/parsers/libunwind/AddressSpace.hpp
@@ -37,7 +37,6 @@
 #include <mach-o/getsect.h>
 #include <mach-o/dyld_priv.h>
 #include <mach/i386/thread_status.h>
-#include <Availability.h>
 
 #include "FileAbstraction.hpp"
 #include "libunwind.h"