The ornamental design for a hair trimmer, as shown and described.
FIG. 1 is a perspective view of a hair trimmer showing my new design;
FIG. 2 is another perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a left elevational view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top side view thereof;
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is a partial enlarged view of FIG. 1.
The broken lines illustrate portions of the hair trimmer and illustrate the boundaries of the enlarged partial view. None of the broken lines form part of the claimed design.