The ornamental design for a skiboot, as shown and described.
1.1 is a first side view of a ski boot;
1.2 is a front view of the ski boot of 1.1;
1.3 is a second side view of the ski boot of 1.1;
1.4 is a rear view of the ski boot of 1.1;
1.5 is a top view of the ski boot of 1.1; and
1.6 is a bottom view of the ski boot of 1.1.
The ornamental design that is claimed is shown in solid lines in the drawings. The broken lines represent portions of the ski boot that form no part of the claimed design.