The ornamental design for a thermostat, as shown and described.
FIG. 1 is a right front perspective view of a thermostat showing my new
design;
FIG. 2 is a right side elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a front elevational view of another embodiment of my new design
of FIG. 1, all other views being substantially similar to FIGS. 1-4, the
rear of both embodiments being substantially flat, plain, and conventional
.