libgltf: fix Linux-with-check build

It seems some of the warnings are treated as errors
in Linux-with-check builds.

Change-Id: I9ed876ba634d944c022838a625164f06f100a7df
(cherry picked from commit a2648efa6748b782bf739c55cf93fb69d547ead1)
Reviewed-on: https://gerrit.libreoffice.org/9980
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>