openvpn: build with --enable-iproute2

OpenVPN fails to build on Debian testing because
net-tools is no longer provided by default.

/reviewed-on nexedi/slapos!534

component/openvpn/buildout.cfg

@@ -16,6 +16,7 @@ md5sum = 4ad8a008e1e7f261b3aa0024e79e7fb7
 configure-options =
   --disable-dependency-tracking
   --disable-plugin-auth-pam
+  --enable-iproute2
 environment =
   PATH=${xz-utils:location}/bin:%(PATH)s
   LZO_LIBS=-L${lzo:location}/lib -llzo2