The ornamental design for a headlamp, as shown and described.
FIG. 1 is a perspective view of a headlamp according to my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a back elevation view thereof;
FIG. 4 is a top side elevation view thereof;
FIG. 5 is a bottom plan elevation view thereof;
FIG. 6 is a left side elevation view thereof; and,
FIG. 7 is a right side view thereof.
The broken lines are directed to environment and are for illustrative purposes only. The broken lines form no part of any embodiment of the claimed design.