Fix warning:

libavformat/nut.c: In function ‘ff_nut_free_sp’:
libavformat/nut.c:80: warning: passing argument 4 of ‘av_tree_enumerate’ from incompatible pointer type
./libavutil/tree.h:92: note: expected ‘int (*)(void *, void *)’ but argument is of type ‘void (*)(void *, void *)’

Originally committed as revision 22659 to svn://svn.ffmpeg.org/ffmpeg/trunk