clients/meson:build: Conditionally build dmabuf-feedback client

As mesa includes gbm_bo_get_fd_for_plane() from 21.1.0 version onwards,
build the dma-buf feedback client only after that. This should provide
some sanity for package maintainers, as this would need pulling a
rather newer mesa version to build it (which might not be available).

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>