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
1 file changed