Do not use '__decay' reserved keyword (fixes build on latest macOS)

[falkTX]

Fixes the issue reported in https://community.vcvrack.com/t/issue-building-valley-on-macos-ventura-13-5-2/20886

Best to not use variables with "__" prefix as those are meant for the compiler and language specifics, but that would require changing quite some stuff so best you do it when you have time.

For now this PR just allows to build your plugin in new systems.