avutil: install des.h, rc4.h and tree.h as public headers

avutil: install des.h, rc4.h and tree.h as public headers

These headers contain functions supposed to be public.

libavutil/des.h:
 av_des_alloc
 av_des_crypt
 av_des_init
 av_des_mac
libavutil/rc4.h:
 av_rc4_alloc
 av_rc4_crypt
 av_rc4_init
libavutil/tree.h
 av_tree_destroy
 av_tree_enumerate
 av_tree_find
 av_tree_insert
 av_tree_node_alloc
 av_tree_node_size

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>