The ornamental design for a funnel cap, as shown and described.
FIG. 1 is a left side view showing my funnel cap, the right side the mirror image of the left side.
FIG. 2 is a front view showing my new design.
FIG. 3 is a rear view thereof.
FIG. 4 is a top view thereof; and,
FIG. 5 is a bottom view thereof.
The dot-dash-dot broken lines in FIGS. 2 and 5 denote the boundaries of the claim and form no part thereof, and the evenly spaced broken lines and the areas within them in FIGS. 2 and 5 depict portions of the funnel cap that form no part of the claimed design.