meta-agl-core/recipes-graphics/wayland: Do not export seat related functions

meta-agl-core/recipes-graphics/wayland: Do not export seat related functions

We're only using the seat initialization parts for
waltham-transmitter-plugin so add them only there. So far there's only a
user for it (the transmitter-plugin) so it is easier to just build it
conditionally for now.

Bug-AGL: SPEC-3879

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I3af5578c319c361babf3fc5497517f2b075a4271
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/26244
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>