graphics/epsonscan2-non-free-plugin: Remove warning, extend pkg-message

There are two problems about using .warning in Makefile.

1. It is evaluated only when make(1) is invoked. So user of binary package never see the message.
2. It is not related to specific target. So the message is displayed with improper case such as make index.

So add a message to pkg-message rather than using .warning in Makefile.

PR: 263056

Approved by: J.R. Oldroyd <fbsd@opal.com> (maintainer)