tpm: fix header for modular build

Fix build for TCG_TPM=m.  Header file doesn't handle this
and incorrectly builds stubs.

drivers/char/tpm/tpm.c:720: error: redefinition of 'tpm_pcr_read'
include/linux/tpm.h:35: error:previous definition of 'tpm_pcr_read' was here
drivers/char/tpm/tpm.c:752: error: redefinition of 'tpm_pcr_extend'
include/linux/tpm.h:38: error:previous definition of 'tpm_pcr_extend' was here

Repairs linux-next's

commit d6ba4521
Author: Mimi Zohar <zohar@linux.vnet.ibm.com>
Date:   Mon Oct 26 09:26:18 2009 -0400

    tpm add default function definitions

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Rajiv Andrade <srajiv@linux.vnet.ibm.com>
Cc: Mimi Zohar <zohar@us.ibm.com>
Cc: James Morris <jmorris@namei.org>
Cc: Eric Paris <eparis@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Morris <jmorris@namei.org>