Check for getaddrinfo in the winsock headers.

This makes us link directly to getaddrinfo if it is available, making
binaries incompatible with anything prior to windows XP. If compatibility
is desired, compile with --target-os=mingw32-prexp.

Patch by Martin Storsjö <martin at martin dot st>.

Originally committed as revision 21442 to svn://svn.ffmpeg.org/ffmpeg/trunk