Revert "Move dckeytab.so installation from debian/rules to debian/samba.install"

This reverts commit 38ce540a.

And fixes:
W: samba source: binaries-have-file-conflict python-samba samba usr/lib/python2.7/dist-packages/samba/dckeytab.x86_64-linux-gnu.so
W: samba source: binaries-have-file-conflict python-samba-dbgsym samba-dbgsym usr/lib/debug/.build-id/33/db5dfe88e544fc39b2a791ee53f1df5f157dbe.debug
W: samba source: intra-source-package-circular-dependency python-samba samba samba-common-bin