Fix build warning about unused argument.

Change-Id: I3ae26dfc4ffd97d039386f55e3290a6cf36ae17d
diff --git a/system/osi/test/alarm_test.cpp b/system/osi/test/alarm_test.cpp
index 438b3c7..d08cf6d 100644
--- a/system/osi/test/alarm_test.cpp
+++ b/system/osi/test/alarm_test.cpp
@@ -4,6 +4,7 @@
 
 extern "C" {
 #include "alarm.h"
+#include "osi.h"
 #include "semaphore.h"
 }
 
@@ -48,7 +49,7 @@
     }
 };
 
-static void cb(void *data) {
+static void cb(UNUSED_ATTR void *data) {
   ++cb_counter;
   semaphore_post(semaphore);
 }