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

Reviewed By: ldionne, #libc

Spies: pcwang-thead, libcxx-commits

Differential Revision: https://reviews.llvm.org/D151375
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 1902140..0f52da9 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -1805,6 +1805,7 @@
 #include <__algorithm/pstl_fill.h>
 #include <__algorithm/pstl_find.h>
 #include <__algorithm/pstl_for_each.h>
+#include <__algorithm/pstl_merge.h>
 #include <__algorithm/pstl_transform.h>
 #include <__algorithm/push_heap.h>
 #include <__algorithm/ranges_adjacent_find.h>