The ornamental design for a backpack, as shown and described.
FIG. 1 is a perspective view of a backpack, which embodies our design.
FIG. 2 is a front view thereof; and,
FIG. 3 is a top view thereof.
The broken lines crossing claimed surfaces depict stitching and form no part of the claim. The broken lines encircling features like straps, buckles, shoulder harness and logos depicts portions of the article and form no part of the claim. The surfaces and features encircled by broken lines form no part of the claim.