clock_adjtime: Use __nonnull to avoid null pointer

clock_adjtime()/clock_adjtime64()
Add __nonnull((2)) to avoid null pointer access.

Link: https://sourceware.org/bugzilla/show_bug.cgi?id=27662
Link: https://sourceware.org/bugzilla/show_bug.cgi?id=29084


Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>