I claim the ornamental design for a toy building element, as shown and described.
FIG. 1 is a front view of a toy building element showing my new design, the rear view being identical;
FIG. 2 is a left side view of the toy building element, the right side view being identical;
FIG. 3 is a top view of the toy building element;
FIG. 4 is a bottom view of the toy building element;
FIG. 5 is an elevated top, front, and side view of the toy building element; and,
FIG. 6 is an elevated bottom, front, and side view of the toy building element.
The tonal shading shown is strictly utilized to illustrate the shape, contours and forms of the toy building element and forms no part of the claimed design.