x64 detection missing an include

Summary: rwspinlock.h was missing the portability.h header which made the x64 detection go wonky, but only shows up when used with isolated rwspinlock use

Test Plan: fbmake runtests

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D1284387

Blame Revision: D1282140

folly/RWSpinLock.h

@@ -106,6 +106,8 @@ pthread_rwlock_t Read        728698     24us       101ns     7.28ms     194us
 
 */
 
+#include "folly/Portability.h"
+
 #if defined(__GNUC__) && !defined(__clang__) && \
   (defined(__i386) || FOLLY_X64 || \
    defined(ARCH_K8))