The ornamental design for a humidifier, as shown and described.
FIG. 1 is a front elevational view of a humidifier showing my new design;
FIG. 2 is a rear elevational view thereof; and
FIG. 3 is a left side view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a perspective view of a humidifier showing my new design;
FIG. 8 is another perspective view thereof; and,
FIG. 9 is a partial enlarged view of FIG. 6.
The dashed lines in FIGS. 6 and 9 represent an enlarged area. The dashed lines in the figures illustrate the portions of the design that form no part of the claimed design. None of the broken lines form any part of the claimed design.