multimedia/mlt: Fixes post r416546 commit

• USE_GNOME=cairo only applies when SWFDEC option is enabled
• USE_XORG=x11 applies even with all options disabled (due to SDL)
• USE_GNOME=glib20 is required when JACK or SWFDEC options are enabled
• Fix pkg-plist: filter_audiospectrum.yml only installed if QT4 enabled

Reported by: mat (mentor)
Reviewed by: mat (mentor)
Approved by: adamw (mentor), portmgr (blanket approval - build fixes)
Differential Revision: https://reviews.freebsd.org/D6776