games/jaggedalliance2: Fix LICENSE_DISTFILES

This port, like many other Rust ports, brings numerous dependencies to
the distinfo, and by default, LICENSE_DISTFILES applies to all of them.
However, due to license permission constraints, it was removing about
800 distfiles with make clean-restricted, which are also used by many
other ports. With this change, the license constraint will apply only to
the port itself.

Approved by: portmgr (blanket)