[libc++][PSTL] Implement std::is_partitioned

Reviewed By: #libc, ldionne

Spies: ldionne, libcxx-commits

Differential Revision: https://reviews.llvm.org/D152853
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index fefa98a..29e7871 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -1816,6 +1816,7 @@
 #include <__algorithm/pstl_find.h>
 #include <__algorithm/pstl_for_each.h>
 #include <__algorithm/pstl_generate.h>
+#include <__algorithm/pstl_is_partitioned.h>
 #include <__algorithm/pstl_merge.h>
 #include <__algorithm/pstl_replace.h>
 #include <__algorithm/pstl_stable_sort.h>