Vector: Fix missing tab images on Special:Preferences

This fixes an issue in vector when you go to special preference the
images would not load. That is because you needed to add ../ to the
file path.

This follows up If248781e44300039eb52262d03b2f6722838dd12

Change-Id: Iee3303eb1288ad5fc9e5a39e4854774a6da375ff