scripts: dtc-version: Don't show error messages

Prevent the error messages produced by which(1), such as the one quoted
below, from being visible in the build outputs.

    which: no dtc in (./scripts/dtc)

This makes the build outputs look a tiny bit cleaner.

Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Reviewed-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>