[package] update libupnp to 1.6.8

[package] update libupnp to 1.6.8

this patch updates libupnp to 1.6.8.

There are lots of fixes since 1.6.6, and 3 concerning embedded
devices:

- fix segfault when there is no services on embedded devices
- customize the stack size of the threads used by pupnp
- fix several memory leaks

And a security issue:
- fix denial-of-service

Signed-off-by: Raphael Huck <rhk@cksum.org>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@24066 3c298f89-4303-0410-b956-a3cf2f4a3e73