The ornamental design for a side panel, as shown and described.
FIG. 1 is a front elevation view of my new side panel design;
FIG. 2 is a right-side elevation view thereof;
FIG. 3 is a left-side elevation view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a front right perspective view thereof;
FIG. 6 is a front left perspective view thereof;
FIG. 7 is a rear top perspective view thereof;
FIG. 8 is a rear bottom perspective view thereof;
FIG. 9 is a bottom plan view thereof; and,
FIG. 10 is a rear elevation view thereof.
The broken lines in the figures are for illustrative purposes only and represent non-claimed environmental subject matter. Neither the portions of the invention shown in broken lines nor the broken lines themselves form any part of the claimed design.