Add void in place of empty function arguments.

Since this is C89 we need to maintain this ancient practice.

Change-Id: I7223e7c38a35cf551b6e3c9159d2e21ebf7e62be
Reviewed-on: https://boringssl-review.googlesource.com/2631
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/digest/digests.c b/crypto/digest/digests.c
index 52d446f..ac00ed3 100644
--- a/crypto/digest/digests.c
+++ b/crypto/digest/digests.c
@@ -234,7 +234,7 @@
 
 struct nid_to_digest {
   int nid;
-  const EVP_MD *(*md_func)();
+  const EVP_MD* (*md_func)(void);
 };
 
 static const struct nid_to_digest nid_to_digest_mapping[] = {