The ornamental design for a hairdrier, as shown and described.
FIG. 1 is a front perspective view of the hair drier showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left view thereof where the right view is a mirror image;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a back perspective view thereof.
The broken lines depict portions of the hair drier that form no part of the claimed design. The dash-dot broken lines define the bounds of the claimed design and form no part thereof.