The ornamental design for a headlamp, as shown and described.
FIG. 1 is a front perspective view embodying the inventive design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a top elevational view thereof;
FIG. 4 is a left-side elevational view thereof;
FIG. 5 is a right-side elevational view thereof;
FIG. 6 is a bottom elevational view thereof; and,
FIG. 7 is a rear perspective view thereof.
The broken lines represent the bounds of the claimed design or unclaimed subject matter and are for illustrative purposes only. The broken lines form no part of the claimed design.