The ornamental design for a hairdryer, as shown and described.
FIG. 1 is a perspective view of a hairdryer showing my new design;
FIG. 2 is another perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan, view thereof.
The broken away symbols in the drawings indicate the appearance of any portion of the hairdryer between the break lines forms no part of the claimed design.