Splitting Earth Wire in Attic for Light
In the US code was recently changed so that if your equipment is fed from the same panel you can take an appropriately sized ground to the ungrounded circuit.
I usually find lighting circuits on 15 amp all receptacles on 20 amp. 
If your lighting circuit is a 15 amp breaker you can tap any ground , if a 20 amp circuit you need to find a 20 amp, and you can use a receptacle or any larger ground to ground this fixture.