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: Kurt Roeckx <kurt@openssl.org>
diff --git a/crypto/x509/x509_vfy.c b/crypto/x509/x509_vfy.c
index 0ec5ca8..85aa113 100644
--- a/crypto/x509/x509_vfy.c
+++ b/crypto/x509/x509_vfy.c
@@ -119,6 +119,7 @@
 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,