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; and,
FIG. 8 is a bottom plan view thereof.
The broken lines shown on claimed surfaces depict stitching and form no part of the claimed design. All other broken lines depict portions of the backpack and form no part of the claimed design.