Makefile.inc1: Silence an error in make packages

Silence an error in "packages" target, like:

eval: /usr/src: Permission denied

on systems without subversion present.

Reviewed by: emaste
Approved by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33153

(cherry picked from commit aaa4cd7f3c933950582dfb4f0eac23a200dafe53)