target-s390x: Mark check_privileged() as !CONFIG_USER_ONLY

The function check_privileged() is only used in the softmmu configs;
wrap it in an #ifndef CONFIG_USER_ONLY to avoid clang warnings on the
linux-user builds.

[rth: Remove inline marker too; it was only there to prevent exactly
this warning in GCC.]

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Message-id: 1419373100-17690-2-git-send-email-peter.maydell@linaro.org
Signed-off-by: Richard Henderson <rth@twiddle.net>
