The ornamental design for a foldingstock for a firearm, as shown and described.
FIG. 1 is a first front isometric view of a folding stock for a firearm showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a second front isometric view thereof.
The broken lines in the Figures show portions of the folding stock for a firearm that form no part of the claimed design. The relatively thin lines in the Figures represent contour and not surface ornamentation.