lsm: don't yet account for IMA in LSM_CONFIG_COUNT calculation

Since IMA is not yet an LSM, don't account for it in the LSM_CONFIG_COUNT
calculation, used to limit how many LSMs can invoke security_add_hooks().

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
[PM: subject line tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>