Guard against missing UNUSED.

apps/db_dump/stubs.c

@@ -6,6 +6,10 @@
 #include "mosquitto_internal.h"
 #include "util_mosq.h"
 
+#ifndef UNUSED
+#  define UNUSED(A) (void)(A)
+#endif
+
 struct mosquitto *context__init(mosq_sock_t sock)
 {
 	UNUSED(sock);