test build of 'Host shield' in minimal env.

protocol/lufa/lufa.h

@@ -47,8 +47,19 @@
 #include <string.h>
 #include <LUFA/Version.h>
 #include <LUFA/Drivers/USB/USB.h>
+#include "host.h"
 
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern host_driver_t lufa_driver;
+
+#ifdef __cplusplus
+}
+#endif
+
 /* extra report structure */
 typedef struct {
     uint8_t  report_id;