Include portability/SysTypes.h for ssize_t

Summary: Required for Windows with the more strict include order via upcoming formatting diffs.

Reviewed By: yfeldblum

Differential Revision: D25636437

fbshipit-source-id: 0a5fd400f63220400f89affb11b66e9692e9b352

folly/detail/Iterators.h

@@ -20,6 +20,8 @@
 #include <iterator>
 #include <type_traits>
 
+#include <folly/portability/SysTypes.h>
+
 /*
  * This contains stripped-down workalikes of some Boost classes:
  *