[SYCL] Fix initialization issue on Windows (#2632)

Using initializer list causes compilation issue on Windows when using
compile.py script.

Author: againull

sycl/include/CL/sycl/detail/spinlock.hpp

@@ -33,7 +33,7 @@ class SpinLock {
   void unlock() { MLock.clear(std::memory_order_release); }
 
 private:
-  std::atomic_flag MLock{ATOMIC_FLAG_INIT};
+  std::atomic_flag MLock = ATOMIC_FLAG_INIT;
 };
 } // namespace detail
 } // namespace sycl