wayland-info: Destroy zwp_linux_dmabuf_feedback_v1 proxy on exit

The dmabuf_feedback proxy was leaked, causing the following runtime
warning with recent Wayland libraries:

warning: queue 0x15c8390 destroyed while proxies still attached:
zwp_linux_dmabuf_feedback_v1@14 still attached

Make sure to destroy the dmabuf_feedback proxy along with the rest of
the dmabuf info.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Closes: #8
Fixes: 240cb739 - wayland-info: Add support for linux_dmabuf version 4