radv: add missing u_atomic.h include
Fixes: 7568c97d ("radv: Use atomics to read query results.")
/cc @bnieuwenhuizen
not sure why it builds on master (I'm guessing some header includes something that ends up including u_atomic.h
), but it doesn't build on 20.1 so I've had to add this header to the backport commit, and I think it should be there on master too, following the include-what-you-use principle :)