UBUNTU: [debian] Use dh_prep instead of 'dh_clean -k'

UBUNTU: [debian] Use dh_prep instead of 'dh_clean -k'

dh_prep needs to be run only once at the root of the
binary-indep dependency chain, i.e., install-headers.

Similarly, dh_testdir and dh_testroot only need to be run once at
the root of the binary-indep dependency chain.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>