main/postmarketos-initramfs: explicitly depend on libinput-libs

!5000 (merged) added a new implicit dependency on libinput, because it specified libinput files in the initramfs file hook. mkinitfs considers it fatal if it cannot find files listed in a file hook.

Later this could be 'optimized' by adding a -libinput subpkg and an install_if=libinput-libs to add this file hook so it's not installed everywhere... but the size increase isn't too much for UIs.. someone can do this later.

libinput-libs-1.26.0-r0 installed size: 476 KiB

This fixes a missing dependency problem where a UI (e.g. "none") doesn't normally depend on libinput, and mkinitfs would fail because these files were missing.