debian: package manpage for cephfs-top

debian: package manpage for cephfs-top

the build already produces usr/share/man/man8/cephfs-top.8, and the
fedora spec ships it via the cephfs-top package. on debian,
debian/cephfs-top.install was missing it, so the file was sitting in
debian/tmp unclaimed and dh_missing complained:

  dh_missing: warning: usr/share/man/man8/cephfs-top.8 exists in
  debian/tmp but is not installed to anywhere

claim the manpage in debian/cephfs-top.install so it lands in the
cephfs-top deb. silences the warning and gives debian users the same
`man cephfs-top` fedora users already have.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>