[#860] Makefile: use protoc-gen-go-grpc in protoc target

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

Makefile

@@ -71,7 +71,9 @@ protoc:
 		echo "⇒ Processing $$f "; \
 		protoc \
 			--proto_path=.:./vendor:/usr/local/include \
-			--go_out=plugins=grpc,paths=source_relative:. $$f; \
+			--go_out=. --go_opt=paths=source_relative \
+			--go-grpc_opt=require_unimplemented_servers=false \
+			--go-grpc_out=. --go-grpc_opt=paths=source_relative $$f; \
 	done
 	rm -rf vendor