A few missing includes

Summary: These were required to build folly on OSX.

Test Plan: build

Reviewed By: davejwatson@fb.com

FB internal diff: D1256856

folly/wangle/ManualExecutor.cpp

@@ -17,6 +17,7 @@
 #include "ManualExecutor.h"
 
 #include <string.h>
+#include <string>
 
 #include <stdexcept>
 

folly/wangle/WangleException.h

@@ -17,6 +17,7 @@
 #pragma once
 
 #include <exception>
+#include <string>
 
 namespace folly { namespace wangle {
 

folly/wangle/detail.h

@@ -16,9 +16,10 @@
 
 #pragma once
 
+#include <atomic>
 #include <folly/Optional.h>
 #include <stdexcept>
-#include <atomic>
+#include <vector>
 
 #include "Try.h"
 #include "Promise.h"