package/ltrace: directly use s.b.o to fetch the archive

During the migration from alioth to gitlab, the git repository for ltrace
was not migrated. There is a repository on gitlab.com, owned by the debian
maintainer, but that repository does not contain the sha1 we know of:
    https://gitlab.com/cespedes/ltrace



s.b.o. is the only known location so far to host the archive, so switch
to it.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>