Fix / suppress new unused warnings for mingw+clang

Bug: 69933068
Test: mmma system/core
Change-Id: I089166a979d3d8c5ada38a7745d507b555048499
diff --git a/libcutils/threads.cpp b/libcutils/threads.cpp
index beea8bd..a7e6b2d 100644
--- a/libcutils/threads.cpp
+++ b/libcutils/threads.cpp
@@ -84,7 +84,7 @@
 
 void   thread_store_set( thread_store_t*          store,
                          void*                    value,
-                         thread_store_destruct_t  destroy )
+                         thread_store_destruct_t  /*destroy*/ )
 {
     /* XXX: can't use destructor on thread exit */
     if (!store->lock_init) {