The ornamental design for a pillowcover, as shown and described.
1. Pillow cover
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Left
1.5 : Right
1.6 : Top
1.7 : Bottom
The broken line portions are shown to illustrate environment and seam and form no part of the claimed design; the dashed-dotted broken lines shown in fig. 1.1 and figs. 1.6 to 1.7 depict a boundary between the claimed portion and the unclaimed portion and form no part of the claimed design.