The ornamental design for a barbell, as shown and described.
FIG. 1 is a top, front, right perspective view of a barbell showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken lines immediately adjacent to the shaded area define the bounds of the claimed design and form no part thereof. The broken lines depicting the remainder of the barbell show features that form no part of the claimed design. The broken line cross-hatching in FIGS. 1-7 forms no part of the claimed design.