x11-wm/jwm: Make SVG option depend on CAIRO option

It seems that only enabling SVG option isn't enough for SVG-based
icon themes to work, so add SVG_IMPLIES=CAIRO.

Reported by: fernandel (on forums.freebsd.org)
URL: https://forums.freebsd.org/threads/jwm-pkg.103838/
Fixes: 4e3c1e294511 ("x11-wm/jwm: Enable SVG option by default")
Approved by: osa, vvd (Mentors, implicit)