We claim the ornamental design for a desk lamp, as shown and described.
FIG. 1 is a perspective view of a desk lamp showing our new design;
FIG. 2 is a perspective fragmentary view therefore showing the light sources;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a rear view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The desk lamp is shown broken away to indicate that no particular length is claimed. The portions of the desk lamp shown in broken lines form no part of the claimed design.