Add missing include in RandomTest.cpp

Summary:
[Folly] Add missing `#include <sys/wait.h>` in `folly/test/RandomTest.cpp`.

Fixes https://github.com/facebook/folly/issues/1371.

Reviewed By: Orvid

Differential Revision: D21910410

fbshipit-source-id: 31a045960b2e67282fa77595dfdf840c338ef661

folly/test/RandomTest.cpp

@@ -26,6 +26,10 @@
 
 #include <folly/portability/GTest.h>
 
+#ifndef _WIN32
+#include <sys/wait.h>
+#endif
+
 using namespace folly;
 
 TEST(Random, StateSize) {