The ornamental design for seating unit, as shown and described.
1. Seating unit
1.1 : Front
1.2 : Back
1.3 : Top
1.4 : Bottom
1.5 : Left
1.6 : Right
1.7 : Cross-sectional view
1.8 : Cross-sectional
1.9 : Enlarged
Portions of the reproductions shown in broken lines are not part of the claimed design. Reproduction 1.7 is a cross-sectional view of the backrest of the seating unit horizontally sectioned from a higher portion of the backrest. Reproduction 1.8 is a cross-sectional view vertically sectioned at the center of the seating unit. Reproduction 1.9 is an enlarged view of reproduction 1.8.