tpm: fix regression caused by section type conflict of tpm_dev_release() in ppc builds

The 81198078 commit reintroduced a regression
(error: __ksymtab_tpm_dev_release causes a section type conflict) that was fixed by commit
cbb2ed4a.
Fix it for good by adding the prototype to tpm.h so sparse doesn't
complain about it anymore.

Reported-by: Tony Camuso <tcamuso@redhat.com>
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>