lint: fix mdlint problem (#3848)
lint: fix mdlint problem (#3848)

This pins the mdl version to 0.9.0 thus avoiding the failing 0.10.0
netsec-ethz/scion,Oncilla/scion,Oncilla/scion,Oncilla/scion,netsec-ethz/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion