package/qemu: use a system-wide slirp

Use a system-wide slirp now that we switched to the up to date
https://gitlab.freedesktop.org/slirp/libslirp



qemu already depends on libglib2 so we don't need to add any new
dependencies

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