package/slirp: switch official tarball

Other "official" tarballs don't ship .tarball-version resulting in a build
failure: https://gitlab.freedesktop.org/slirp/libslirp/-/issues/24

Fixes:
 - http://autobuild.buildroot.org/results/0b9cff1bc650876a6fff6102b2cb31dcdf4c5e8f



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>