libweston/desktop: Migrate libweston-desktop/libweston-desktop.h

With commit 'Move libweston-desktop into libweston' we've moved out
libweston-desktop DSO into libweston.  Move also the header to
libweston/desktop.

This removes removes the libweston-desktop pc file and bumps libweston
major version to 12.

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