(rx) Include types.h first

Summary: We need to do forward declarations first, and types.h already does declarations, so just make sure it gets included first in each file.

Test Plan:
tests

Reviewed By: davejwatson@fb.com

Subscribers: trunkagent, fugalh, exa, folly-diffs@, jsedgwick, yfeldblum

FB internal diff: D1861040

Tasks: 6258585

Signature: t1:1861040:1428604059:de6393e3c87e0694a6b04ad6d440cc9635a1008d

folly/wangle/rx/Observable.h

@@ -16,9 +16,9 @@
 
 #pragma once
 
+#include <folly/wangle/rx/types.h> // must come first
 #include <folly/wangle/rx/Subject.h>
 #include <folly/wangle/rx/Subscription.h>
-#include <folly/wangle/rx/types.h>
 
 #include <folly/RWSpinLock.h>
 #include <folly/SmallLocks.h>

folly/wangle/rx/Observer.h

@@ -16,7 +16,7 @@
 
 #pragma once
 
-#include <folly/wangle/rx/types.h>
+#include <folly/wangle/rx/types.h> // must come first
 #include <functional>
 #include <memory>
 #include <stdexcept>

folly/wangle/rx/Subject.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <folly/wangle/rx/types.h> // must come first
 #include <folly/wangle/rx/Observable.h>
 #include <folly/wangle/rx/Observer.h>
 

folly/wangle/rx/Subscription.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <folly/wangle/rx/types.h> // must come first
 #include <folly/wangle/rx/Observable.h>
 
 namespace folly { namespace wangle {