Fix compile warning on iobuf.h
Fix compile warning on iobuf.h

Summary:
Following compiple warning fixed:

```
/home/ubuntu/fbthrift/thrift/lib/py3/iobuf.h:25:23: warning: extra tokens at end of #ifdef directive
 #ifdef PY_VERSION_HEX < 0x03040000
                       ^
```

Before this change PyGILState_Check() will always be overridden (I am assuming that is the intention of this line) even if we are using Python 3.4 or higher.

Reviewed By: yfeldblum

Differential Revision: D12999012

fbshipit-source-id: d8fa8ec48a8c8914aa70a049f571299c95d4af9b

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift