input: bump wl_seat version to 6

input: bump wl_seat version to 6

New in version 6 are touch shape, touch orientation and axis source
wheel tilt. Weston doesn't support any of them yet but simply not
sending the new events and new enum value is sufficient to claim to
support this version.

Also bump the Wayland requirement to 1.17 to ensure both version 6 and 7
definitions are in the XML.

The reason for bumping to v6 without implementing the new features is
that we must support v7 to make use of struct ro_anonymous_file
introduced in the previous commit.

Signed-off-by: Sebastian Wick <sebastian@sebastianwick.net>