Makefile.inc1: sysent: allow subordinate sysent targets to run in parallel

makesyscalls.lua (and indeed makesyscalls.sh) are both safe to be run in
parallel, so let's do it.

This is a trivial difference because runtime per-target is pretty small, but
I like seeing it run in parallel when my muscle memory types make -sj4.

Reviewed by: brooks, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26594