package/xen: add slirp dependency for tools

package/xen: add slirp dependency for tools

Build of xen tools fails if slirp is built before xen because xen is not
compatible with spice slirp which does not provide libslirp.h:

/home/buildroot/autobuild/instance-2/output-1/build/xen-4.13.0/tools/qemu-xen/net/slirp.c:40:10: fatal error: libslirp.h: No such file or directory
 #include <libslirp.h>
          ^~~~~~~~~~~~

Indeed, xen prefers a system-provided slirp over its internal one

So add slirp as a mandatory dependency (now that we switched to the up
to date https://gitlab.freedesktop.org/slirp/libslirp)

This build failure is raised since, at least, version 4.13.0

Fixes:
 - http://autobuild.buildroot.org/results/b80b33ed558518f7bbb0a3c8586bf2d0b8acc36f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>