fdo-mime.eclass: Mark the eclass as deprecated

fdo-mime.eclass: Mark the eclass as deprecated

The GNOME team has committed the xdg-utils.eclass serving exactly
the same purpose as fdo-mime.eclass, supposedly with the goal of
replacing it. However, it seems that they have never bothered to
actually hint the deprecation in the fdo-mime.eclass in any way.
As a result, developers are still adding references to this eclass
instead of using xdg-utils or xdg, and/or not working towards replacing
them.

Add an explicit deprecation notice to the fdo-mime.eclass to make it
clear that the eclass should not be used in new packages, and what
the replacement eclasses are.