ring_buffer: Use std::atomic_size_t in a static assert

Avoids the need to repeat "std::" twice

src/common/ring_buffer.h

@@ -30,7 +30,7 @@ class RingBuffer {
     static_assert(capacity < std::numeric_limits<std::size_t>::max() / 2 / granularity);
     static_assert((capacity & (capacity - 1)) == 0, "capacity must be a power of two");
     // Ensure lock-free.
-    static_assert(std::atomic<std::size_t>::is_always_lock_free);
+    static_assert(std::atomic_size_t::is_always_lock_free);
 
 public:
     /// Pushes slots into the ring buffer