USB: fix build error in the USB core if CONFIG_PROCFS is disabled

Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

include/linux/proc_fs.h

@@ -193,6 +193,7 @@ static inline void proc_net_remove(const char *name)
 
 #define proc_root_driver NULL
 #define proc_net NULL
+#define proc_bus NULL
 
 #define proc_net_fops_create(name, mode, fops)  ({ (void)(mode), NULL; })
 #define proc_net_create(name, mode, info)	({ (void)(mode), NULL; })