The ornamental design for a lampholder, as shown and described.
FIG. 1 is a right, front, top perspective view of a lamp holder showing my new design;
FIG. 2 is a left, rear, bottom 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.
In the drawings, the broken lines illustrate portions of the lamp holder that form no part of the claimed design. The unshaded surfaces form no part of the claimed design.