pam: Password is const in setcred

pam: Password is const in setcred

afs_setcred.c gets the "password" pointer from pam_get_data, which
always gives a const pointer (unlike pam_get_item used in afs_auth.c
&c, which sometimes gives a const or not-const pointer, depending on
the PAM implementation).

So, declare password const, to get better type checking.

Reviewed-on: http://gerrit.openafs.org/4553
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 94a9b2afd82b6729ddceb7ef736ddeb039e0ae1b)

Change-Id: I3171babfbdf29e7aa543a17f7dd543deedc9b30c
Reviewed-on: http://gerrit.openafs.org/6294
Tested-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Derrick Brashear <shadow@dementix.org>