send warning to stderr. (#53)

With --batch json there must not be any other output on stdout, so redirect warnings to stderr will show the warning on the console and only the json output is on stdout.

spectre-meltdown-checker.sh

@@ -122,7 +122,7 @@ _echo_nol()
 
 _warn()
 {
-	_echo 0 "\033[31m${@}\033[0m"
+	_echo 0 "\033[31m${@}\033[0m" >&2
 }
 
 _info()