configure: inlcude sys/types.h when checking sys/socket.h

Silly BSD systems need this despite POSIX requiring nothing of the
kind.  Any system with sys/socket.h should also have sys/types.h,
so this is unlikely to break anything, and we already do this in
another place.

Originally committed as revision 21326 to svn://svn.ffmpeg.org/ffmpeg/trunk
