_STD -> _VSTD to avoid macro clash on windows

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@134190 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/system_error.cpp b/src/system_error.cpp
index caafc32..763d62c 100644
--- a/src/system_error.cpp
+++ b/src/system_error.cpp
@@ -147,7 +147,7 @@
             what_arg += ": ";
         what_arg += ec.message();
     }
-    return _STD::move(what_arg);
+    return _VSTD::move(what_arg);
 }
 
 system_error::system_error(error_code ec, const string& what_arg)