libweston: bump major to 6

At least the following commits break the libweston ABI, strictly speaking:
- a5630eaf
- ce62cb3d
- 195dadeb
- c18ffd39
- ea54c2fd
- 27d7c395
- acff29b3
- 67629674
- c9c24773
- 65e1be12

Furhtermore, 64fbd0f4 break the weston
executable exported ABI.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>