wireguard: allowedips: add missing __rcu annotation to satisfy sparse

A __rcu annotation got lost during refactoring, which caused sparse to
become enraged.

Fixes: bf7b042d

 ("wireguard: allowedips: free empty intermediate nodes when removing single node")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>