exec=on for /usr/src after base/head r359260

Update 2020-03-30:

r359426 fixed this issue. We may again set exec=off or have noexec set in our /etc/fstab.

If you mount /usr/src as noexec or have set the ZFS exec property to off, now is the time to allow executables to be run. r359260 added contrib/kyua/doc/manbuild.sh, which is run as part of make buildworld in base/head.