update sources to Ubuntu-4.15.0-51.55

(generated with debian/scripts/import-upstream-tag)

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

diff --git a/submodules/ubuntu-bionic b/submodules/ubuntu-bionic
index 410b374872d719c06d22dc5d735b100f65199273..18e103f8c89eef5b91b6741643e331000c7359d8 160000 (submodule)
--- a/submodules/ubuntu-bionic
+++ b/submodules/ubuntu-bionic
@@ -1 +1 @@
-Subproject commit 410b374872d719c06d22dc5d735b100f65199273
+Subproject commit 18e103f8c89eef5b91b6741643e331000c7359d8