package/libostree: bump to version 2020.3

package/libostree: bump to version 2020.3

Upstream only ever made a dot-relase once, and it is not even available
as a tag, and it is not even listed as a separate release on their
github release page. We can assume that dot-release is not in upstream
habits. Drop the VERSION_MAJOR variable, just use VERSION.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
[yann.morin.1998@free.fr:
  - drop VERSION_MAJOR
  - two spaces in hash file
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>