crypto: api - Fix Kconfig dependencies for FIPS

Currently FIPS depends on MODULE_SIG, even if MODULES is disabled.
This change allows the enabling of FIPS without support for modules.

If module loading support is enabled, only then does
FIPS require MODULE_SIG.

Signed-off-by: Alec Ari <neotheuser@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>