update submodule to what will be 3.1.0

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

diff --git a/ifupdown2 b/ifupdown2
index 288a88d3e432c312c684c435eff9104be1f22953..c2b847f7afc3f45524973662f78f2cc76a2bc507 160000 (submodule)
--- a/ifupdown2
+++ b/ifupdown2
@@ -1 +1 @@
-Subproject commit 288a88d3e432c312c684c435eff9104be1f22953
+Subproject commit c2b847f7afc3f45524973662f78f2cc76a2bc507