The ornamental design for a skullcap, as shown and described.
FIG. 1 is an enlarged front perspective view of our ornamental design for a skull cap.
FIG. 2 is a left side elevation view of our ornamental design shown in FIG. 1.
FIG. 3 is a right side elevation view of our ornamental design shown in FIG. 1.
FIG. 4 is a front elevation view of our ornamental design shown in FIG. 1.
FIG. 5 is a back elevation view of our ornamental design shown in FIG. 1; and,
FIG. 6 is a top plan view of our ornamental design shown in FIG. 1.
The partial human figure shown in broken lines in the drawings is included to illustrate how the skull cap may appear when worn and forms no part of the claimed design.