d/rules: enable slirp again

Commit d03e1b3 ("update submodule and patches to 7.2.0") argued that
slirp is not explicitly supported in PVE, but that is not true. In
qemu-server, user networking is supported (via CLI/API) when no bridge
is set on a virtual NIC. So slirp needs to stay to keep such NICs
working.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>

debian/rules

@@ -62,7 +62,6 @@ ${BUILDDIR}/config.status: configure
 	--disable-smartcard \
 	--disable-strip \
 	--disable-xen \
-	--disable-slirp \
 	--enable-curl \
 	--enable-docs \
 	--enable-glusterfs \
@@ -75,6 +74,7 @@ ${BUILDDIR}/config.status: configure
 	--enable-opengl \
 	--enable-rbd \
 	--enable-seccomp \
+	--enable-slirp \
 	--enable-spice \
 	--enable-usb-redir \
 	--enable-virglrenderer \