An ornamentaldesign for a hair trimmer, as shown and described.
FIG. 1 is a front overhead isometric view of the hair trimmer;
FIG. 2 is a rear overhead isometric view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof;
FIG. 7 is a rear view thereof;
FIG. 8 is a front view thereof;
FIG. 9 is a perspective view thereof in an open configuration; and,
FIG. 10 is a left side view thereof in an open configuration.
The dash-dot-dash broken lines shown along the bottom of the hair trimmer illustrate portions of the hair trimmer that form no part of the claimed design.