ospf6d: fix build problem with ancient json-c versions

Some CI VMs are using really old versions of json-c (pre 2013 [1])
that expect filenames to be passed as "char *" instead of "const char *".

Add some explicit casts to fix the resulting compiler errors on those
VMs (passing "char *" when the API expects "const char *" is fine).
Hopefully this commit should be reverted once the CI is updated to use
newer versions of json-c.

[1] https://github.com/json-c/json-c/commit/20e4708c

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

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