multimedia/mlt-qt5 needs USE_GL = gl

[phryk]

multimedia/mlt-qt5 fails building with poudriere saying "need to specify
gl component with USE_GL".

I assume this only happens when the OpenGL option is activated for
multimedia/mlt, but haven't tested.

Adding "USE_GL = gl" to multimedia/mlt-qt5's Makefile fixes the issue for me,
tho I expect that's not a one-size-fits-all fix.


PS: Is this even the right place to report issues like that?