java/openjdk25: Add jre_headless flavor

Add flavor to make a headless jre package, where dev tools and x11
support is removed. As requested in Bug #266059.

PR: 266059
Reviewed by: fuz, jrm
Approved by: fuz (Mentor), jrm
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D53707