unlike in Git, signing commits is has no use for SVN.
In Git, you can rewrite the history using doing a forced push. In SVN, revisions are immutable.
You *can* change revisions in SVN if you have root access on the server and access to the filesystem there. And you have to be able to take the repo offline while you want to change the existing revisions.
But if you have that kind of access, signing a commit won't help anyway...