[libc++] Use explicit #include instead of transitive #include (#115420)

This benchmark test currently uses `std::unique_ptr` without explicitly
`#include <memory>`. I think we should not rely on transitive inclusion.

libcxx/test/benchmarks/vector_operations.bench.cpp

@@ -13,6 +13,8 @@
 #include <cstring>
 #include <deque>
 #include <functional>
+#include <memory>
+#include <string>
 #include <vector>
 
 #include "benchmark/benchmark.h"