fwupd: address fwupd_release_get_uri deprecation warnings

fwupd_release_get_uri() was deprecated in favour of
fwupd_release_get_locations() with 1.5.6. The latter takes a GPtrArray,
instead of a simple const gchar*, so need need some extra boilerplate.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>