xdg-shell: add missing enum attribute to resize

This helps binding generators such as the one in wayland-rs.

Signed-off-by: Ivan Molodetskikh <yalterz@gmail.com>
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Status Job ID Name Coverage
  Containers Build
passed #1657946
container_build

00:00:24

 
  Test
passed #1657947
test

00:00:50