The ornamental design for a headlamp, as shown and described.
FIG. 1 is a perspective view of a first embodiment of my headlamp;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom view thereof;
FIG. 5 is a right side view thereof; and,
FIG. 6 is a left side view thereof.
The dashed lines in the figures represent environmental structure only and form no part of the claimed design. The long-short dashed lines in the figures represent boundaries between claimed and unclaimed surfaces and form no part of the claimed design.