ccan/container_of/container_of.h: avoid warning with -Wcast-qual

* ccan/container_of/container_of.h (container_off_var):
  avoid warning with -Wcast-qual
  [ccan ba5ad771af4aa9e085498de6c3c665c52694460f (Rusty Russell)]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46548 b2dd03c8-39d4-4d8f-98ff-823fe69b080e