Support EVP_PKEY_{sign,verify}_message with Ed25519.

It's amazing how short p_ed25519.c is.

BUG=187

Change-Id: Ib2a5fa7a4acf2087ece954506f81e91a1ed483e1
Reviewed-on: https://boringssl-review.googlesource.com/14449
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/evp.h b/include/openssl/evp.h
index 4ff8412..cb2731e 100644
--- a/include/openssl/evp.h
+++ b/include/openssl/evp.h
@@ -841,5 +841,6 @@
 #define EVP_R_UNSUPPORTED_ALGORITHM 128
 #define EVP_R_UNSUPPORTED_PUBLIC_KEY_TYPE 129
 #define EVP_R_NOT_A_PRIVATE_KEY 130
+#define EVP_R_INVALID_SIGNATURE 131
 
 #endif  /* OPENSSL_HEADER_EVP_H */