crypto: skcipher - Add comment for skcipher_alg->base

This patch adds a missing comment for the base parameter in struct
skcipher_alg.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/skcipher.h

@@ -115,6 +115,7 @@ struct crypto_skcipher {
  *	    IV of exactly that size to perform the encrypt or decrypt operation.
  * @chunksize: Equal to the block size except for stream ciphers such as
  *	       CTR where it is set to the underlying block size.
+ * @base: Definition of a generic crypto algorithm.
  *
  * All fields except @ivsize are mandatory and must be filled.
  */