The ornamental design for a backpack, as shown and described.
FIG. 1 is a front perspective view of a backpack showing my new design;
FIG. 2 is a rear 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; and,
FIG. 8 is a bottom plan view thereof.
The dash dot dash lines depict handles, straps and lines and form no part of the claim. The evenly dashed lines depict stitching and form no part of the claim.