systemd: fix 0019-*.patch

This was introduced in https://github.com/NixOS/nixpkgs/pull/99621, but
didn't follow the process documented in
pkgs/os-specific/linux/systemd/default.nix, namely, the `git am` and
`git format-patch` workflow, which caused
`0019-revert-get-rid-of-seat_can_multi_session.patch` to not apply with
`git am` due to missing authorship information.

I did apply this patch manually, and copied authorship information from
4e384ddc11.

pkgs/os-specific/linux/systemd/0019-logind-seat-debus-show-CanMultiSession-again.patch

@@ -1,3 +1,13 @@
+From 3999d8949ddaf9296928f603661abcea13576d83 Mon Sep 17 00:00:00 2001
+From: Thomas Tuegel <ttuegel@mailbox.org>
+Date: Mon, 26 Oct 2020 21:21:38 +0100
+Subject: [PATCH 19/19] logind-seat-debus: show CanMultiSession again
+
+Fixes the "switch user" function in Plasma < 5.20.
+---
+ src/login/logind-seat-dbus.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
 diff --git a/src/login/logind-seat-dbus.c b/src/login/logind-seat-dbus.c
 index a91765205c..742aeb1064 100644
 --- a/src/login/logind-seat-dbus.c
@@ -11,3 +21,6 @@ index a91765205c..742aeb1064 100644
          SD_BUS_PROPERTY("CanTTY", "b", property_get_can_tty, 0, SD_BUS_VTABLE_PROPERTY_CONST),
          SD_BUS_PROPERTY("CanGraphical", "b", property_get_can_graphical, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
          SD_BUS_PROPERTY("Sessions", "a(so)", property_get_sessions, 0, 0),
+-- 
+2.28.0
+

pkgs/os-specific/linux/systemd/default.nix

@@ -88,7 +88,7 @@ in stdenv.mkDerivation {
     ./0016-systemd-sleep-execute-scripts-in-etc-systemd-system-.patch
     ./0017-kmod-static-nodes.service-Update-ConditionFileNotEmp.patch
     ./0018-path-util.h-add-placeholder-for-DEFAULT_PATH_NORMAL.patch
-    ./0019-revert-get-rid-of-seat_can_multi_session.patch
+    ./0019-logind-seat-debus-show-CanMultiSession-again.patch
   ];
 
   postPatch = ''