build-sys: declare rootfs.tar.xz as dep of rootfs/
build-sys: declare rootfs.tar.xz as dep of rootfs/

This forces make to re-extract the tarball when it's changed

Distrotech/kmod,lpereira/kmod,lucasdemarchi/kmod,lucasdemarchi/kmod,lucasdemarchi/kmod,Distrotech/kmod,pmmccorm/kmod,pmmccorm/kmod,Distrotech/kmod,lpereira/kmod,pmmccorm/kmod,lpereira/kmod