Building block for a toy building set

FIG. 1 is a top plan view of my ornamental design for a building block for a toy building set;

FIG. 2 is a top, left perspective view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a front side view thereof;

FIG. 6 is a bottom plan view thereof; and,

FIG. 7 is a bottom, rear perspective view thereof.

The broken lines in the drawings depict portions of the building block for a toy building set that form no part of the claimed design.