The ornamental design for a jacket with built-in backpack, as shown and described.
FIG. 1 is a rear perspective view of a jacket with built-in backpack;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left view thereof; the right view being a mirror image;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines showing the wearer, fastening straps and buckles are included for the purpose of illustrating environmental structure and form no part of the claimed design. The broken lines shown in the jacket represent stitching in the fabric.