[PUSHED][PATCH] Typo fix "usefull -> useful"

[Caolán McNamara]

On Mon, 2013-02-25 at 22:57 -0400, Gregg King wrote:
> Typo fix "usefull -> useful"
> 
> 
> Bug 60724
> https://bugs.freedesktop.org/show_bug.cgi?id=60724

Pushed by Bjoern on Monday. Thanks for this.

C.