Log Message:
Remove the include of xextensions buildlink3.mk.

Its code doesn't reference any x11/extensions/*.h headers.

And building it doesn't seem to need it either.

If it was needed before, then probably one of its other dependencies ended
up needing it and that is the one that should include it in its own
buildlink3.mk file.

(I am looking at these because I am working on updating xextensions
package which includes a rename to xextproto.)

Okayed by maintainer.

No revision change as it was a "build" dependency.