selinux: constify source policy in cond_policydb_dup()

cond_policydb_dup() duplicates conditional parts of an existing policy.
Declare the source policy const, since it should not be modified.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
[PM: various line length fixups]
Signed-off-by: Paul Moore <paul@paul-moore.com>