The ornamental design for a backpack, as shown and described.
FIG. 1 is a perspective 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.
The features presented in broken lines represent unclaimed portions of the backpack, as well as unclaimed stitching and unclaimed surface ornamentation on the backpack and form no part of the claimed design.