devel/subversion: Enable the GPG_AGENT option by default

gpg-agent is one of the easiest ways of storing credentials in Subversion.
Let's enable support for it by default to make life easier for Subversion
users, who do not want to depend on gnome-keyring or kwallet for password
storing.

PR: 244280
Approved by: lev (maintainer)
Differential Revision: https://reviews.freebsd.org/D23849