The ornamental design for an airfoil, as shown and described.
FIG. 1 is a top perspective view of the present airfoil;
FIG. 2 is a front view of same;
FIG. 3 is a left side view of the airfoil of FIG. 1, both sides being mirror images of each other;
FIG. 4 is a rear view of same;
FIG. 5 is a top view of the present airfoil; and,
FIG. 6 is a bottom view of same.
In the drawings, the dash-dot-dot-dash broken lines in FIG. 1 depict a supporting structure for an airfoil, while the evenly spaced broken lines in FIG. 1 indicate the outer edge of the indicated support structure. The broken lines form no part of the claimed design.