backend-rdp: unref keymap after associating with seat

The function weston_seat_init_keyboard makes sure that it has its
own reference to keymap, hence we can safely drop our reference.
This is similarly done in the X11 backend. It avoids leaking a
struct xkb_keymap per connection.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Status Job ID Name Coverage
  Container Prep
passed #373354
container_prep

00:00:29

 
  Build
passed #373355
build-native-meson

00:00:29