gnucash: enable dbi

My accounts are stored in the DBI backend (SQLite, specifically).
Before the present change, GnuCash would be compiled without DBI
 support (`--disable-dbi` flag), so I could only use the XML backend.
Now I can use either backend.

I made the change only for 2.6. It looks like 2.4 works the same way,
so maybe I'll enable DBI for that one later.

Making nix packages is delightful! It might replace my accounting hobby!