Remove deprecated UNUSED macro (5/5)

Include osi.h for UNUSED_ATTR.

Test: mma -j32

Change-Id: I43260669dc1f54639e46cc9620093d727ee86276
diff --git a/stack/gatt/gatt_sr.cc b/stack/gatt/gatt_sr.cc
index 075829d..a4aa238 100644
--- a/stack/gatt/gatt_sr.cc
+++ b/stack/gatt/gatt_sr.cc
@@ -24,6 +24,7 @@
 
 #include "bt_target.h"
 #include "bt_utils.h"
+#include "osi/include/osi.h"
 
 #if (BLE_INCLUDED == TRUE)
 #include <string.h>