add inline to copy_within

[PSeitz]

In lz4_flex a fixed size parameter is used to copy_within to avoid calls to libc memmove.
However, I can see in the call stack still see calls to libc. I think it's because of the missing inline.

It's using the equivalent of this:
https://godbolt.org/z/cKqYYvKbT
(in this example copy_within is getting inlined)

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