lnrpc: add search path to gen_protos.sh

Without this addition, sub servers cannot import google annotations
in their proto files.

lnrpc/gen_protos.sh

@@ -28,6 +28,7 @@ do
     echo "Generating protos from ${file}, into ${DIRECTORY}"
 
     protoc -I/usr/local/include -I.\
+           -I$GOPATH/src/github.com/grpc-ecosystem/grpc-gateway/third_party/googleapis \
            --go_out=plugins=grpc,paths=source_relative:. \
            ${file}
 done