usb_kbd: fix usb_kbd_deregister when DEVICE_DEREGISTER not enable

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Remy Böhmer <linux@bohmer.net>
diff --git a/include/devices.h b/include/devices.h
index 6b78d58..20ddfc4 100644
--- a/include/devices.h
+++ b/include/devices.h
@@ -91,7 +91,9 @@
  */
 int	device_register (device_t * dev);
 int	devices_init (void);
+#ifdef CONFIG_SYS_DEVICE_DEREGISTER
 int	device_deregister(char *devname);
+#endif
 struct list_head* device_get_list(void);
 device_t* device_get_by_name(char* name);
 device_t* device_clone(device_t *dev);