build: avoid pod2man on tarball

build: avoid pod2man on tarball

virt-sanlock-cleanup.8 has static contents (no dependency on
configure), but is generated by pod2man (a perl dependency that
maintainers must have, but which ordinary tarball users need
not have).  Therefore, ensure that it is always part of the
tarball, even though it is only conditionally installed.

This is similar to commit 6db98a2d4b, but made simpler by the fact
that the .8 page is static content.

* tools/Makefile.am (EXTRA_DIST): Add virt-sanlock-cleanup.8.