Don't explicitly derive Eq for enums

I got bitten by this bug: tokio-rs/prost#332
when I added the enum `ShieldedProtocol` to the file `service.proto`.
The problem is that `prost` implicitly derives `Eq` for enums, so
deriving it explicitly via `type_attribute` causes a conflict. Lukily,
there is another method `message_attribute` that operates only on
messages and not enums.

zebrad/build.rs

@@ -46,7 +46,7 @@ fn main() {
         // The lightwalletd gRPC types don't use floats or complex collections,
         // so we can derive `Eq` as well as the default generated `PartialEq` derive.
         // This fixes `clippy::derive_partial_eq_without_eq` warnings.
-        .type_attribute(".", "#[derive(Eq)]")
+        .message_attribute(".", "#[derive(Eq)]")
         .compile(
             &["tests/common/lightwalletd/proto/service.proto"],
             &["tests/common/lightwalletd/proto"],