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>

scripts/dtc-version.sh

@@ -15,7 +15,7 @@ if [ ${#dtc} -eq 0 ]; then
 	exit 1
 fi
 
-if ! which $dtc >/dev/null ; then
+if ! which $dtc > /dev/null 2>&1 ; then
 	echo "Error: Cannot find dtc: $dtc"
 	exit 1
 fi