FIG. 1 is a perspective view of a sprinkler base showing my new design; FIG. 2 is a top plan view thereof; FIG. 3 is a front elevational view showing my new design; and, FIG. 4 is a left side elevational view thereof. The bottom view is unornamented and forms no part of the claimed design. The left side view is a mirror image of the right side view.