libslirp: Add the slirp library as this is required for the net user backend in qemu

libslirp: Add the slirp library as this is required for the net user backend in qemu

- Looking through some of the changelog and some mail list communications it looks like
   qemu decided they did noty want to maintain their own bundled version of libslirp when
   the majority of OS's had their own version now in place. Ubuntu 18.04 did not have
   libslirp but qemu stopped supporting that version from qemu-7.1
- So it looks like all OS's have a standard libslirp available now and qemu have taken
   the decision to no longer have their own version but to use the system version. That
   was always possible to do if use of the system version was explicitly defined but
   the default was to use the bundled version.
- No evidence that libslirp is deprecated.
- The last version of libslirp was released a year ago but it looks like every month or
   so there are a couple of commits merged. The last was a month ago.

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>