proto: update to grpc-gateway v1.8.6

lnrpc/gen_protos.sh

@@ -11,7 +11,7 @@ protoc -I/usr/local/include -I. \
 # Generate the REST reverse proxy.
 protoc -I/usr/local/include -I. \
        -I$GOPATH/src/github.com/grpc-ecosystem/grpc-gateway/third_party/googleapis \
-       --grpc-gateway_out=logtostderr=true:. \
+       --grpc-gateway_out=logtostderr=true,paths=source_relative:. \
        rpc.proto
 
 # Finally, generate the swagger file which describes the REST API in detail.