The ornamental design for a waistband, as shown and described.
FIG. 1 is a top perspective view of the waist band showing my new design;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a left side view of my design;
FIG. 6 is a right side view of my design;
FIG. 7 is a front elevational view of my design with the rear elevational view being a mirror image thereof;
FIG. 8 is a top perspective view showing my design; and,
FIG. 9 is an environmental view showing my design in use on a body shown in broken lines holding a small electrical device shown in broken lines for illustrative purposes only and forms no part of the claimed design.