Add dyn to error::Error impl function cause
Add dyn to error::Error impl function cause

This fixes the warning trait objects without an explicit `dyn` are deprecated,
which causes builds with warnings-as-errors to fail. The error can be
triggered in standalone rsdparsa builds by setting
RUSTFLAGS="-D bare-trait-objects" when invoking cargo build.

nils-ohlmeier/rsdparsa