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 another perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof;
FIG. 8 is a bottom plan view thereof;
FIG. 9 is a perspective view of the backpack where the backpack is in a closed state; and,
FIG. 10 is an enlarged view of portion 10 shown in FIG. 1.
The broken lines in the drawings depict portions of the backpack that form no part of the claimed design. The unevenly dashed lines in FIG. 1 and FIG. 10 depict the cutline of the sectional view and form no part of the claim.