Fix 'Please include winsock2.h before windows.h' warnings with mingw32

* random/rndw32.c: include winsock2.h before windows.h.
* src/ath.h [_WIN32]: Ditto.
* tests/benchmark.c [_WIN32]: Ditto.

Patch silences warnings of following type:
/usr/lib/gcc/i686-w64-mingw32/4.6/../../../../i686-w64-mingw32/include/winsock2.h:15:2: warning: #warning Please include winsock2.h before windows.h [-Wcpp]

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>