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.

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

(cherry picked from commit 0dbb00733c4a177a9a22d61a627d377d0876ea16)
(cherry picked from commit 10ef3bf823c5d5db0303eb1552f1faa47cb0e58c)