Ensure compatibility with old liblz4

WWrequested to merge
heilkitty/openmw:old-liblz4-fix into master

Ubuntu 18.04 still uses liblz4 r131, so the build fails.

see: https://discord.com/channels/260439894298460160/260443579908882434/768127775759794187

All the credit goes to @Capostrophic .

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