The meson port changed the effectsdir in the .pc file to
$(pkgdatadir)/gnome-video-effects, resulting in paths like
…/share/gnome-video-effects/gnome-video-effects.

Here we fix the regression such that the path matches to
the actual path where the files are installed.
Also we use ${prefix} pkg-config variable in the path
so that it can be easily overridden.