ustr: bump debian patch to 1.0.4-6

Version 1.0.4-5 is no longer available, replace with 1.0.4-6 so ustr can
continue to be built.

[Peter: use snapshot.debian.org as suggested by Baruch]
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>