mingw64 compilation fix

Implemented patch by 'anguista' to fix mingw64 compilation, found on this forum post from Feb 2010: http://ffmpeg.arrozcru.org/forum/viewtopic.php?f=8&t=1225&p=4874#p4874

My builds for mingw (win32), Linux (32- and 64-bit) and OS X (32- and 64-bit) all still work after this patch as well.