nixos/sddm: fix unit: add RequiresMountsFor for state directory

This helps correctness for systems which need to mount particular
directories (impermanence, preservation). This option has no effect if
the system does not need to mount directories / if no mountpoints exist
for this directory or its parents.

Signed-off-by: Nicolas Dumazet <nicdumz.commits@gmail.com>

nixos/modules/services/display-managers/sddm.nix

@@ -439,6 +439,10 @@ in
           "plymouth-quit.service"
           "systemd-logind.service"
         ];
+        # sddm stores state in this directory, which should be mounted.
+        unitConfig.RequiresMountsFor = [
+          config.users.users.sddm.home
+        ];
       };
     };
   };