The ornamental design for a shirtpocket, as shown and described.
FIG. 1 is a front view of pocket showing my new design positioned on a representative shirt shown in dashed lines.
FIG. 2 is a perspective view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof; and,
FIG. 5 is a left side view thereof.
The dotted lines in FIG. 1 and FIG. 2 shown on the surface of the pocket represent stitching and are part of the claimed design. The dashed lines, illustrating an article of clothing, show the pocket in use and form no part of the claimed design.