update sources to Ubuntu-4.15.0-87.87

(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 a8c1e6affa4262808b062e3422686a85efa541b5..4e6a0a8c30072c29266823fd0784080e98f9c254 160000 (submodule)
--- a/submodules/ubuntu-bionic
+++ b/submodules/ubuntu-bionic
@@ -1 +1 @@
-Subproject commit a8c1e6affa4262808b062e3422686a85efa541b5
+Subproject commit 4e6a0a8c30072c29266823fd0784080e98f9c254