Add missing dependency on rpm2cpio

It is needed by the RPM loader, introduced in swh.loader.core v5.2.0

I'm sending this MR to the debian/unstable-swh branch; is it the right one?

debian/control

@@ -24,6 +24,7 @@ Build-Depends: debhelper (>= 9),
                python3-tenacity,
                python3-toml,
                opam,
+               rpm2cpio,
                zstd,
 Standards-Version: 3.9.6
 Homepage: https://forge.softwareheritage.org/diffusion/60/