kvs1025: fix missing prototype for kv_usb_escape()

backend/kvs1025_usb.h

@@ -16,6 +16,11 @@ SANE_Bool kv_usb_already_open (PKV_DEV dev);
 void kv_usb_close (PKV_DEV dev);
 void kv_usb_cleanup (void);
 
+SANE_Status
+kv_usb_escape (PKV_DEV dev,
+	       PKV_CMD_HEADER header,
+	       unsigned char *status_byte);
+
 SANE_Status kv_usb_send_command (PKV_DEV dev,
 				 PKV_CMD_HEADER header,
 				 PKV_CMD_RESPONSE response);