The ornamental design for the head support, as shown and described.
FIG. 1 is a top view of a head support showing my new design;
FIG. 2 is a bottom view of a head support showing my new design;
FIG. 3 is a right side elevation end view of a head support showing my new design;
FIG. 4 is a left side elevation end view of a head support showing my new design;
FIG. 5 is a back view of a head support showing my new design;
FIG. 6 is a front view of a head support showing my new design; and,
FIG. 7 is a perspective view of a head support showing my new design.
The broken lines are included for the purpose of illustrating unclaimed portions of the head support in the form of the two curved rectangular panels and stitching. The broken lines in the drawings form no part of the claimed design.