Merge pull request #1472 from bwhmather/X509_REQ_digest

add binding for openssl `X509_REQ_digest`
diff --git a/cryptography/hazmat/bindings/openssl/x509.py b/cryptography/hazmat/bindings/openssl/x509.py
index b74c118..95bc7c3 100644
--- a/cryptography/hazmat/bindings/openssl/x509.py
+++ b/cryptography/hazmat/bindings/openssl/x509.py
@@ -122,6 +122,8 @@
 int X509_REQ_set_pubkey(X509_REQ *, EVP_PKEY *);
 int X509_REQ_sign(X509_REQ *, EVP_PKEY *, const EVP_MD *);
 int X509_REQ_verify(X509_REQ *, EVP_PKEY *);
+int X509_REQ_digest(const X509_REQ *, const EVP_MD *,
+                    unsigned char *, unsigned int *);
 EVP_PKEY *X509_REQ_get_pubkey(X509_REQ *);
 int X509_REQ_print_ex(BIO *, X509_REQ *, unsigned long, unsigned long);