The ornamental design for a headlamp, as shown and described.
FIG. 1 is a Front elevational view of a headlamp showing my new design;
FIG. 2 is a Rear elevational view thereof;
FIG. 3 is a Left side view thereof;
FIG. 4 is a Right side view thereof;
FIG. 5 is a Top plan view thereof;
FIG. 6 is a Bottom plan view thereof;
FIG. 7 is a Perspective views thereof;
FIG. 8 is another Perspective views from back side thereof; and,
FIG. 9 is an enlarged detail view of the encircled region of FIG. 4.
The dot-dash broken lines in FIGS. 4 and 9 are for annotative purpose only and form no part of the claimed design. The dash-dash lines throughout the drawing figures depict portions of the headlamp that form no part of the claimed design.