haikuwebkit: force libwebp version.
haikuwebkit: force libwebp version.

We don't want to be using an old one, so force use of the 0.6.x branch.
Should help fix a build failure on buildbots.

Begasus/haikuports,haikuports/haikuports,haikuports/haikuports,nielx/haikuports,moochris/haikuports,Begasus/haikuports,nielx/haikuports,haikuports/haikuports,moochris/haikuports,haikuports/haikuports,Begasus/haikuports,Begasus/haikuports,nielx/haikuports,Begasus/haikuports,haikuports/haikuports,moochris/haikuports