The new, original, and ornamental design for a headlamp for a vehicle, as shown.
FIG. 1 is a perspective view of the headlamp for a vehicle, showing our design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left view thereof, the right view being a mirror image thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The broken line showing of the other portion of the claimed design is included for the purpose of illustrating the other portions of the claimed design and forms no part of the claimed design.