Add a custom %clean implementation

Let's make sure we clean up after ourselves. We have to remove
the generated timeout user config file, the file list files and the
generated .lang file.

systemd.spec

@@ -1166,5 +1166,11 @@ fi
 
 %files standalone-shutdown -f .file-list-standalone-shutdown
 
+%clean
+rm -rf $RPM_BUILD_ROOT
+rm -f 10-timeout-abort.conf.user
+rm -f .file-list-*
+rm -f %{name}.lang
+
 %changelog
 %autochangelog