[libc++][NFC] Minor reformatting in <cstddef>

Author: ldionne

libcxx/include/cstddef

@@ -66,8 +66,8 @@ using ::max_align_t _LIBCPP_USING_IF_EXISTS;
 _LIBCPP_END_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 17
-namespace std // purposefully not versioned
-{
+namespace std { // purposefully not versioned
+
 enum class byte : unsigned char {};
 
 _LIBCPP_HIDE_FROM_ABI inline constexpr byte operator|(byte __lhs, byte __rhs) noexcept {
@@ -127,7 +127,6 @@ template <class _Integer, __enable_if_t<is_integral<_Integer>::value, int> = 0>
 }
 
 } // namespace std
-
-#endif
+#endif // _LIBCPP_STD_VER >= 17
 
 #endif // _LIBCPP_CSTDDEF