crypto: sig - Move crypto_sig_*() API calls to include file

The crypto_sig_*() API calls lived in sig.c so far because they needed
access to struct crypto_sig_type:  This was necessary to differentiate
between signature algorithms that had already been migrated from
crypto_akcipher to crypto_sig and those that hadn't yet.

Now that all algorithms have been migrated, the API calls can become
static inlines in <crypto/sig.h> to mimic what <crypto/akcipher.h> is
doing.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>