src/config/keybind.c: fix keybind insertion order

src/config/keybind.c: fix keybind insertion order

This restores the intended behavior of keybinds set by `<default />`
to be overwritten by manually configured keybinds which come later in
the config.

In `src/keyboard.c`, `handle_keybinding()` is going backwards through
the list of keybindings and breaks after the first match.

`wl_list_insert(&list_node, item)` will insert the new item *after* the
list_node so if its called multiple times with the same list_node as
fist argument the result will be a reversed list. Using `list_node.prev`
instead will result in a non-reversed list.