Add missing include in test; NFC. Thanks to Jonathan Wakely for the report.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@284120 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/algorithms/alg.modifying.operations/alg.partitions/is_partitioned.pass.cpp

@@ -14,6 +14,7 @@
 //     is_partitioned(InputIterator first, InputIterator last, Predicate pred);
 
 #include <algorithm>
+#include <functional>
 #include <cassert>
 
 #include "test_iterators.h"