This allows to build with --enable-slirp / -D slirp=enabled, even when
libslirp is not installed on the system.  "meson subprojects download"
will pull it from git in that case.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230302131848.1527460-4-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>