glib: add compatibility interface for g_strcmp0()

This patch fixes compilation errors when building against glib < 2.16.0
due to the missing g_strcmp0() function.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Message-id: 1413457177-10132-1-git-send-email-arei.gonglei@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>