for glibc, if _GNU_SOURCE is defined, <time.h> will include <sys/timex.h>
but not for musl, so add "#include <sys/timex.h>"
although `man clock_adjtime` said "#include <sys/timex.h>", but it won't
work for glibc w/o "#include <time.h>", I don't known why yet.
PS it seems clock_adjtime is used nowhere?
Signed-off-by: Z. Liu <zhixu.liu@gmail.com>