The ornamental design for a facemask, as shown and described.
FIG. 1 is a front perspective view of a facemask showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a left side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a rear perspective view thereof.
The uneven-length broken lines depict environmental structure and form no part of the claimed design. The even-length broken lines represent stitching and form no part of the claimed design.