The ornamental design for a shower headkit, as shown and described.
FIG. 1 is a front elevation view of a shower head kit according to my new design;
FIG. 2 is a rear elevation view thereof;
FIG. 3 is a left side elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a perspective view thereof.
The broken lines showing of a shower head kit are for the purpose of illustrating environmental structures of the shower head kit and form no part of the claimed design.