sntrup: Avoid linking issues in external_override tests

This linking worked only in CI and local builds, but not during
the build in RPM as it fails on missing symbols that were defined
only in the main library. This is solved as with the other digest
dependencies in external crypto by removing the intermediate
function. We are already linking the md_*.o objects.

The error was like this

sh: symbol lookup error: /path/libssh/libssh-0.12.0-build/libssh-0.12.0/redhat-linux-build/lib/libsntrup761_override.so: undefined symbol: crypto_hash_sha512

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Pavol Žáčik <pzacik@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

include/libssh/sntrup761.h

@@ -35,10 +35,6 @@ extern "C" {
 #define HAVE_SNTRUP761 1
 #endif
 
-extern void crypto_hash_sha512(unsigned char *out,
-                               const unsigned char *in,
-                               unsigned long long inlen);
-
 /*
  * Derived from public domain source, written by (in alphabetical order):
  * - Daniel J. Bernstein

src/external/sntrup761.c

@@ -27,9 +27,7 @@ sntrup761_enc (uint8_t *c, uint8_t *k, const uint8_t *pk,
 void
 sntrup761_dec (uint8_t *k, const uint8_t *c, const uint8_t *sk);
 
-extern void crypto_hash_sha512 (unsigned char *out,
-				const unsigned char *in,
-				unsigned long long inlen);
+extern int sha512(const unsigned char *digest, size_t len, unsigned char *hash);
 
 #define MAX_LEN 761
 
@@ -701,7 +699,7 @@ Hash_prefix (unsigned char *out, int b, const unsigned char *in, int inlen)
   x[0] = b;
   for (i = 0; i < inlen; ++i)
     x[i + 1] = in[i];
-  crypto_hash_sha512 (h, x, inlen + 1);
+  sha512 (x, inlen + 1, h);
   for (i = 0; i < 32; ++i)
     out[i] = h[i];
 }

src/sntrup761.c

@@ -37,13 +37,6 @@
 #include "libssh/session.h"
 #include "libssh/ssh2.h"
 
-void crypto_hash_sha512(unsigned char *out,
-                        const unsigned char *in,
-                        unsigned long long inlen)
-{
-    sha512(in, inlen, out);
-}
-
 #ifndef HAVE_LIBGCRYPT
 static void crypto_random(void *ctx, size_t length, uint8_t *dst)
 {