The ornamental design for a support accessory, as shown and described.
FIG. 1 is a top, front, and left side perspective view of a support accessory showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a top plan view thereof, the bottom plan view being a mirror image; and,
FIG. 5 is a left side elevation view thereof, the right side elevation view being a mirror image.
The broken lines represent unclaimed subject matter and form no part of the claimed design.
The claim is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.