Add _NETBSD_SOURCE definition to fix compilation on NetBSD.
NetBSD is unlikely to fix their headers and the FATE box passes it
as -D_NETBSD_SOURCE on the command line anyway. In this case, it's
better to keep the hack well-contained within this file.
Closes issue 886.
patch by Jorge Acereda, jacereda brainstorm es

Originally committed as revision 22164 to svn://svn.ffmpeg.org/ffmpeg/trunk
