gdbus-codegen: Don't assume bindir and datadir share prefix

This assumption breaks when, for instance:

* Called as /bin/gdbus-codegen
* Installed on Windows in a directory that is not `bin/`

For such cases, we cannot make any assumptions about the directory
structure, and must hard-code the datadir.

https://bugzilla.gnome.org/show_bug.cgi?id=786785