Force WITHOUT_FREEBSD_UPDATE when WITHOUT_PORTSNAP is set

freebsd-update depends on phttpget from portsnap. We could move phttpget
out of portsnap and build it as long as WITHOUT_FREEBSD_UPDATE and
WITHOUT_PORTSNAP are not both set, but for now just make the dependency
explicit.

PR: 228220
Reported by: Dries Michiels
Sponsored by: The FreeBSD Foundation