Add a missing USB prototype

Signed-off-by: Stefan Reinauer <stepan@coresystems.de>

Index: include/libpayload.h
===================================================================
--- include/libpayload.h	(revision 3600)
+++ include/libpayload.h	(working copy)
@@ -110,6 +110,15 @@
 /** @} */
 
 /**
+ * @defgroup usb USB functions
+ * @{
+ */
+int usb_initialize(void);                                                      
+/** @} */
+
+
+
+/**
  * @defgroup input Device functions
  * @{ @}
  */
