Add missing virxdrdefs.h include to log_protocol

Commit 2b6f6ad introduced the virxdrdefs.h header with
common definitions to be included in the protocol files,
but logging/log_protocol.x was missed, so add it there as well.

Hopefully this fixes build on OS X.

src/logging/log_protocol.x

@@ -2,6 +2,7 @@
  */
 
 %#include "internal.h"
+%#include "virxdrdefs.h"
 
 typedef opaque virLogManagerProtocolUUID[VIR_UUID_BUFLEN];