logging: Make sure to install standard "end" command in VTY node

When you are in the config-log VTY node, you expect "end" to work
like in any other sub-node of config.
diff --git a/include/osmocom/vty/command.h b/include/osmocom/vty/command.h
index caafdeb..8fbb482 100644
--- a/include/osmocom/vty/command.h
+++ b/include/osmocom/vty/command.h
@@ -361,6 +361,7 @@
 extern struct cmd_element config_exit_cmd;
 extern struct cmd_element config_help_cmd;
 extern struct cmd_element config_list_cmd;
+extern struct cmd_element config_end_cmd;
 char *host_config_file();
 void host_config_set(const char *);