The ornamental design for a humidifier, as shown and described.
FIG. 1 is a perspective view of a humidifier 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 side elevational view thereof;
FIG. 6 is a right side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken lines in the drawings depict portions of the humidifier that form no part of the claimed design. The oblique shading lines in the FIGS. 2 and 4 are included to denote the transparent quality of surface planes on the humidifier.