The ornamental design for a foldingknife, as shown and described.
FIG. 1 is an isometric view of a folding knife showing my new design.
FIG. 2 is a side view of a folding knife.
FIG. 3 is a side view of a folding knife.
FIG. 4 is a bottom view of a folding knife.
FIG. 5 is a top view of a folding knife.
FIG. 6 is a front end view of a folding knife.
FIG. 7 is a back end of a folding knife; and,
FIG. 8 is an isometric view of the folding knife, with its handle in an open configuration.
The broken lines in the drawings have been shown for the purpose of illustrating an interior area and boundary along the outer edges of the handle, and form no part of the claimed design.