noexcept applied to <future>.

llvm-svn: 160607
diff --git a/libcxx/src/future.cpp b/libcxx/src/future.cpp
index 2935711..feb37e4 100644
--- a/libcxx/src/future.cpp
+++ b/libcxx/src/future.cpp
@@ -47,7 +47,7 @@
 }
 
 const error_category&
-future_category()
+future_category() _NOEXCEPT
 {
     static __future_error_category __f;
     return __f;