Move winsock2.h in front of a potential windows.h include.

Files that included FileMap.h (possibly transitively), before including
ByteOrder.h (which pulls in winsock2.h directly), will experience a
compiler warning/error from the latest mingw headers. This happens because
the headers require that winsock2.h come before windows.h in all cases.
The simplest (and most error-proof) fix for now is to include winsock2.h
before this use of windows.h.

Change-Id: I33069e4c9962d9820d0ea5976554f89d7ff6307c