Missing "export" causing interactive prompts

[rphedman]

https://github.com/FDio/vpp/blame/632d9f9dbaf038f75b32bf302345e1a9e7529c26/Makefile#L293

I believe this line is missing the export as below. If DEBIAN_FRONTEND is not set in the environment, it will be interactive. Not using "export" sets it only within the "make", not its subprocesses.

export DEBIAN_FRONTEND=noninteractive

Without this in place I get prompted for iperf3 needing to be restarted during the build.