GCE: Add /usr/src and /usr/ports to METALOG

We ship these in order to comply with GCE Marketplace rules about
providing source code and licenses for all the software we ship as
part of images.

Reviewed by: markj
MFC after: immediately (15.0-RC2)
Differential Revision: https://reviews.freebsd.org/D53796

(cherry picked from commit 0dbb00733c4a177a9a22d61a627d377d0876ea16)