[libc] Allow container_of() to be used on volatile pointers

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/stddef.h

  */
 #define container_of( ptr, type, field ) ( {				\
 	type *__container;						\
-	const typeof ( __container->field ) *__field = (ptr);		\
+	const volatile typeof ( __container->field ) *__field = (ptr);	\
 	__container = ( ( ( void * ) __field ) -			\
 			offsetof ( type, field ) );			\
 	__container; } )