Stackable Building Block

FIG. 1 depicts a three-quarters perspective view of a new and ornamental design for a stackable building block.

FIG. 2 depicts a bottom perspective view thereof of a new and ornamental design for a stackable building block.

FIG. 3 depicts a front elevational view thereof of a new and ornamental design for a stackable building block.

FIG. 4 depicts a rear elevational view thereof of a new and ornamental design for a stackable building block.

FIG. 5 depicts a left side elevational view thereof of a new and ornamental design for a stackable building block.

FIG. 6 depicts a right side elevational view thereof of a new and ornamental design for a stackable building block; and,

FIG. 7 depicts a top plan view thereof of a new and ornamental design for a stackable building block.