lnrpc/signrpc: add new gen_protos.sh to generate minimal protos

In this commit, we add a new proto generation script to match the one in
the main lnrpc package. This script differs, as we don't need to
generate the REST proxy stuff (for now).

lnrpc/signrpc/gen_protos.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+protoc -I/usr/local/include -I. \
+       -I$GOPATH/src \
+       --go_out=plugins=grpc:. \
+       signer.proto