The ornamental design for a siding trim, as shown and described.
FIG. 1 is a front elevational view of a siding trim showing my new design;
FIG. 2 is a rear elevational view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a left elevational view thereof;
FIG. 5 is a right elevational view thereof; and,
FIG. 6 is a bottom plan view thereof.
The siding trim is shown with a symbolic break in the middle. The appearance of any portion of the article between the break lines forms no part of the claimed design.