buildscript: Use git fetch instead of git pull for vlc git

Merged Soomin Lee requested to merge bubu/VLCKit:buildscript/pull/fetch into master

If the vlc directory is already dirty(patches have been applied before) the build will possibly fail on git pull --rebase due conflicts.

If the developer wants to rebase some current work, -n option should be considered.

This patch will be backported to 3.0 if accepted.

Merge request reports