Rename RingBuffer#add for the JVM to not shadow

[kevinoconnor7]

RingBuffer extends from the non-mutable AbstractList, but defines its own add fun which doesn't return any value. For the JVM there will implicitly be a fun add(T): boolean added to the supertype, which means RingBuffer#add is now shadowing it.

Thankfully this is all in a private class so we don't need to worry about a breaking API change.

^KT-55562