Fixup the WDDX fix to compile against older HHVM.

Author: Orvid

hphp/runtime/ext/wddx/ext_wddx.h

@@ -20,6 +20,7 @@
 
 #include "hphp/runtime/ext/extension.h"
 #include "hphp/runtime/base/string-buffer.h"
+#include "hphp/util/either.h"
 
 namespace HPHP {
 

hphp/util/either.h

@@ -169,6 +169,7 @@ struct Either {
    * Returns whether this Either contains neither left nor right.
    */
   bool isNull() const { return !(bits & ~TagBit); }
+  uintptr_t toOpaque() const { return bits; }
 
 private:
   static_assert(