tpm: add stubs

Commit c37cacabf22 moved tpm_cleanup() in the main loop exit, however this
function is not available when compiling with --disable-tpm.

Provides necessary stubs to keep code clean of #ifdef'fery.

Reported-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <20171023102903.256AF7456A0@zero.eik.bme.hu>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
