RT2751: Declare get_issuer_sk() earlier.

Add a declaration for get_issuer_sk() so that other
functions in x509_vf.c could use it.  (Planned work
around cross-certification chains.)
Reviewed-by: NKurt Roeckx <kurt@openssl.org>

crypto/x509/x509_vfy.c

@@ -119,6 +119,7 @@ static int check_trust(X509_STORE_CTX *ctx);
 static int check_revocation(X509_STORE_CTX *ctx);
 static int check_cert(X509_STORE_CTX *ctx);
 static int check_policy(X509_STORE_CTX *ctx);
+static int get_issuer_sk(X509 **issuer, X509_STORE_CTX *ctx, X509 *x);
 
 static int get_crl_score(X509_STORE_CTX *ctx, X509 **pissuer,
 			unsigned int *preasons,