I claim the ornamental design for the wire spool, as shown and described.
FIG. 1 is a front view of the wire spool.
FIG. 2 is a rear view of the claimed design.
FIG. 3 is a side view of the claimed design.
FIG. 4 is a top view of the claimed design.
FIG. 5 is a bottom view of the claimed design; and,
FIG. 6 is a perspective view of the claimed design in which a roll of fencing wire is unwinding thereon, and which is depicted in broken lines as environmental matter forming no part of the claimed design.