Add DXF export to CLI export type

[snhobbs]

Version 2.10.1-dev~ccf6a3
Compiled on Oct 11 2023 at 21:26:44

The DXF export works great from the GUI but from the CLI help:

> ./gerbv.sh --help

...
  -x, --export=<png|pdf|ps|svg|rs274x|drill|idrill>
                          Export a rendered picture to a file with
                          the specified format.

If called on anyway:

> ./gerbv.sh User_1.gbr -x dxf

Unrecognized "dxf" export type.

Looks like the option just isn't included.