Fix -Wunused-private-field

Change-Id: Ib60e8dc2625c0c5e10fa109e452af0bc6174e763

rocclr/thread/semaphore.hpp

@@ -50,14 +50,16 @@ class Semaphore : public HeapObject {
   static constexpr size_t state_size = sizeof(std::atomic_int) +
                                        alignof(std::atomic_int);
 
+  union {
 #ifdef _WIN32
-  void* handle_;  //!< The semaphore object's handle.
-  char padding_[64 - state_size - sizeof(handle_)];
+    void* handle_;  //!< The semaphore object's handle.
 #else  // !_WIN32
-  sem_t sem_;  //!< The semaphore object's identifier.
-  char padding_[64 - state_size - sizeof(sem_)];
+    sem_t sem_;  //!< The semaphore object's identifier.
 #endif /*!_WIN32*/
 
+    char padding_[64 - state_size];
+  };
+
  public:
   Semaphore();
   ~Semaphore();