Bug#908812: libgl1-mesa-dev: dependency on transitional packages

[Jörg-Volker Peetz]

Version: 18.1.9-1

Dear Maintainer(s),

in version 18.1.9-1 this package still depends on the packages 
x11proto-dri2-dev and
x11proto-gl-dev which are marked as transitional.

Regards,
Jörg.