Fixing up some ABI issues

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@134639 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/future b/include/future
index 48b84cf..9f3780b 100644
--- a/include/future
+++ b/include/future
@@ -452,6 +452,8 @@
 
     _LIBCPP_INLINE_VISIBILITY
     const error_code& code() const throw() {return __ec_;}
+
+    virtual ~future_error() _NOEXCEPT;
 };
 
 class __assoc_sub_state