Makefile.inc1: Build source packages before sets

To build set-src, we first need to build the source packages. Add a
.ORDER to ensure this happens. Otherwise, in a parallel build, sets
might be built before the src-* packages have finished building, and
set-src will be mysteriously missing.

MFC after: 3 seconds
Reported by: cperciva
Actually diagnosed by: jrtc27
One-line fix by: ivy
Reviewed by: cperciva
Differential Revision: https://reviews.freebsd.org/D53076