shared/wall: use logind if build without utmp support

If systemd is build without utmp support, use sd_get_sessions() in
utmp_wall() to get a list of logged in users with the corresponding tty.

src/shared/meson.build

@@ -168,6 +168,7 @@ shared_sources = files(
         'verbs.c',
         'vlan-util.c',
         'volatile-util.c',
+        'wall.c',
         'watchdog.c',
         'web-util.c',
         'wifi-util.c',