macosx: Fix VLCLibraryCollectionViewFlowLayout content sizing
This fixes the bottom-most contents of the collection view being inaccessible.
With fix:
Screen_Recording_2022-11-23_at_23.25.04
Before fix:
Screen_Recording_2022-11-23_at_23.28.11
Signed-off-by: Claudio Cambra developer@claudiocambra.com