configure: add <sys/timex.h> for clock_adjtime on musl (#2085)

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>

qemu/configure

@@ -1455,6 +1455,7 @@ fi
 clock_adjtime=no
 cat > $TMPC <<EOF
 #include <time.h>
+#include <sys/timex.h>
 
 int main(void)
 {