Add a missing _VSTD:: before a call to merge. Fixes PR43034. Checked the rest of 'algorithm' looking for unqualified calls. Didn't find any.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369463 91177308-0d34-0410-b5e6-96231b3b80d8

include/algorithm

@@ -4382,7 +4382,7 @@ merge(_InputIterator1 __first1, _InputIterator1 __last1,
 {
     typedef typename iterator_traits<_InputIterator1>::value_type __v1;
     typedef typename iterator_traits<_InputIterator2>::value_type __v2;
-    return merge(__first1, __last1, __first2, __last2, __result, __less<__v1, __v2>());
+    return _VSTD::merge(__first1, __last1, __first2, __last2, __result, __less<__v1, __v2>());
 }
 
 // inplace_merge