The ornamental design for a headgear, as shown.
FIG. 1 is a perspective view of a headgear showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a bottom plan view thereof; and,
FIG. 6 is a side elevational view, the other side being a mirror image thereof.