RODBUS_BACKUP_VERSION not being properly set in cmake

[jadamcrain]

https://github.com/stepfunc/rodbus/blob/f5ae98380b4ca6ab247526ca573021f6fb624cac/ffi/bindings/c/CMakeLists.txt#L5C1-L5C35

Need to set this to the current version and ensure it gets updated for each release.