The ornamental design for an endcap, as shown and described.
FIG. 1 is a perspective view of an endcap;
FIG. 2 is a front elevational view of the endcap of FIG. 1;
FIG. 3 is a rear elevational view of the endcap of FIG. 1;
FIG. 4 is a right side elevational view of the endcap of FIG. 1;
FIG. 5 is a left side elevational view of the endcap of FIG. 1;
FIG. 6 is a top plan view of the endcap of FIG. 1; and,
FIG. 7 is a bottom plan view of the endcap of FIG. 1.
The broken lines in FIGS. 1 and 4-7 represent structure and boundaries that are not part of the claimed design, but rather show the environment in which the design is associated.