Add generic OPERATION_NOT_SUPPORTED error code.

Implementations of ENGINEs often don't want to implement every function.
This change adds an error code for those situations.

Change-Id: Id6b7eace36d06ffad7f347f556d942d447d8a2fd
Reviewed-on: https://boringssl-review.googlesource.com/1920
Reviewed-by: Wan-Teh Chang <wtc@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/engine.h b/include/openssl/engine.h
index 367e98e..4a4f37d 100644
--- a/include/openssl/engine.h
+++ b/include/openssl/engine.h
@@ -100,4 +100,6 @@
 }  /* extern C */
 #endif
 
+#define ENGINE_R_OPERATION_NOT_SUPPORTED 100
+
 #endif  /* OPENSSL_HEADER_ENGINE_H */