build: Use old syntax for fallback dependencies
build: Use old syntax for fallback dependencies

With meson 0.56, this can be done more elegantly with
allow_fallback and a [provide] section in the wrap
files, but with 0.55, we need to explictly mention
the variable names in the meson file.

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf