dtrace: Remove $FreeBSD$ from genmakefiles.sh.

$FreeBSD$ is gone in head, so this should be removed as well.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D41595

(cherry picked from commit 7a3af0a59b56078e88b8009824fdd99e8df3f625)

Approved by: re (cperciva@)