Add missing include in string_view tests. Patch from Billy ONeil @ microsoft

llvm-svn: 285012
diff --git a/libcxx/test/std/strings/string.view/string.view.ops/copy.pass.cpp b/libcxx/test/std/strings/string.view/string.view.ops/copy.pass.cpp
index 0a2ed837..4160146 100644
--- a/libcxx/test/std/strings/string.view/string.view.ops/copy.pass.cpp
+++ b/libcxx/test/std/strings/string.view/string.view.ops/copy.pass.cpp
@@ -19,6 +19,7 @@
 
 
 #include <string_view>
+#include <algorithm>
 #include <cassert>
 
 #include "test_macros.h"