The ornamental design for a lighthouse, as shown and described.
FIG. 1 is a pictorial view of a lighthouse showing my new design.
FIG. 2 is a front elevational view of the lighthouse of FIG. 1.
FIG. 3 is a side elevational view of the lighthouse of FIG. 1.
FIG. 4 is a back elevational view of the lighthouse of FIG. 1.
FIG. 5 is a top view of the lighthouse of FIG. 1; and,
FIG. 6 is a bottom view of the lighthouse of FIG. 1.
The broken line showing of the light structure is included for the purpose of illustrating portions of the lighthouse and forms no part of the claimed design.