The ornamental design for a backpack, as shown and described.
FIG. 1 is an isometric view of a backpack showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof showing my design.
The figures contain two sets of broken lines, a larger set and a smaller set. The larger set of broken lines represent the body of a bag, which is not part of the claimed invention. The smaller set of broken lines represent stitching, which is not part of the claimed invention.