Bug#877555: libgegl-dev should supply VAPI bindings

[Jeremy Bicha]

> The vapigen program needs to be installed on the build machine. The VAPI should then be generated.
> There is a check in configure.ac for vapigen:https://git.gnome.org/browse/gegl/tree/configure.ac#n478
>vapigen is part of the valac package, e.g.https://packages.debian.org/stretch/i386/valac/filelist

gegl already Build-Depends on valac.

Thanks,
Jeremy Bicha