selinux: move debug functions into debug configuration

avtab_hash_eval() and hashtab_stat() are only used in policydb.c when
the configuration SECURITY_SELINUX_DEBUG is enabled.

Move the function definitions under that configuration as well and
provide empty definitions in case SECURITY_SELINUX_DEBUG is disabled, to
avoid using #ifdef in the callers.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>