The ornamental design for a backpack, as shown and described.
FIG. 1 is a front view in perspective of a backpack showing my new design, in which a front flap is folded in a downward oriented position;
FIG. 2 is a front view in perspective of the backpack of FIG. 1 in a configuration in which the front flap is positioned from its downward oriented position to an upward oriented position;
FIG. 3 is a front view of the backpack of FIG. 1;
FIG. 4 is a rear view of the backpack of FIG. 1;
FIG. 5 is a left side view of the backpack of FIG. 1;
FIG. 6 is a right side view of the backpack of FIG. 1;
FIG. 7 is a top view of the backpack of FIG. 1; and,
FIG. 8 is a bottom view of the backpack of FIG. 1.
The broken lines in the figures illustrate portions of the backpack that form no part of the claimed design.