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>
2 jobs for fix-memory-leaks in 1 minute and 51 seconds (queued for 1 second)
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  Container Prep
passed #449571
container_prep

00:00:25

 
  Build
passed #449572
build-native-meson

00:01:25