The ornamental design for a headlamp, as shown and described.
FIG. 1 is a perspective view of my headlamp;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof, the left side view thereof being a mirror image;
FIG. 4 is a top view thereof; and,
FIG. 5 is a bottom side view thereof.
The oblique lines on the lens of my headlamp design indicate that the lens is at least translucent and may be transparent. The dashed lines in the figures form no part of the claimed design. All other surface(s) or portion(s) of my headlamp not shown in the figures or otherwise described in the specification, such as the location behind the lens of the headlamp, form no part of the claimed design.