The ornamental design for a pocketknife, as shown and described.
FIG. 1 is a perspective view of the pocket knife;
FIG. 2 is a top view of the pocket knife;
FIG. 3 is a bottom view of the pocket knife;
FIG. 4 is a left side view of the pocket knife;
FIG. 5 is a right side view of the pocket knife;
FIG. 6 is a back view of the pocket knife; and,
FIG. 7 is a front view of the pocket knife.
The broken lines shown in FIGS. 1-7 depict portions of the folding knife in which the design is embodied that are not considered part of the claimed design.