We claim the ornamental design for a desk lamp, as shown and described.
FIG. 1 is a left-side view of a desk lamp of the claimed design;
FIG. 2 is a right-side view of a desk lamp of the claimed design;
FIG. 3 is a front view of a desk lamp of the claimed design;
FIG. 4 is a rear view of a desk lamp of the claimed design;
FIG. 5 is a bottom view of the horizontal bar of the desk lamp in FIGS. 1-4, 6 and 7;
FIG. 6 is a top view of a desk lamp of the claimed design; and,
FIG. 7 is a top, front, right-side perspective view of a desk lamp of the claimed design.
The ornamental design which is claimed is shown in solid lines in the drawings. The broken lines in the drawings are for illustrative purposes only and form no part of the claimed design.