README.md: Remove the #if/endif from ac_btree_foreach()
README.md: Remove the #if/endif from ac_btree_foreach()

Since commit 98c4ac5 ("ac_btree: Enable ac_btree_foreach_data() for more
systems") ac_btree_foreach_data() is no longer restricted to FreeBSD or
newer versions of glibc.

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@digital-domain.net>

ac000/libac