Restore usr/lib/kernel in systemd

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

debian/systemd-boot.install

@@ -14,6 +14,5 @@ usr/share/bash-completion/completions/bootctl
 usr/share/bash-completion/completions/kernel-install
 usr/share/zsh/vendor-completions/_bootctl
 usr/share/zsh/vendor-completions/_kernel-install
-usr/lib/kernel
 ../../extra/kernel-install.d/* usr/lib/kernel/install.d
 ../../extra/zz_kernel-install etc/initramfs/post-update.d

debian/systemd.install

@@ -17,6 +17,7 @@ usr/lib/sysusers.d/systemd-journal.conf
 usr/lib/sysusers.d/systemd-network.conf
 usr/lib/systemd/
 usr/lib/tmpfiles.d/
+usr/lib/kernel
 <!stage1> usr/lib/*/cryptsetup/
 usr/share/bash-completion/
 usr/share/zsh/vendor-completions/