crypto: cast6 - fix sparse warnings (symbol was not declared, should be static?)

Fix "symbol 'x' was not declared. Should it be static?" sparse warnings.
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/cast6.h

@@ -20,4 +20,9 @@ int cast6_setkey(struct crypto_tfm *tfm, const u8 *key, unsigned int keylen);
 void __cast6_encrypt(struct cast6_ctx *ctx, u8 *dst, const u8 *src);
 void __cast6_decrypt(struct cast6_ctx *ctx, u8 *dst, const u8 *src);
 
+extern const u32 cast6_s1[256];
+extern const u32 cast6_s2[256];
+extern const u32 cast6_s3[256];
+extern const u32 cast6_s4[256];
+
 #endif