configure: Add sys/timex.h to probe clock_adjtime

It is not a part of standard time.h. Glibc put it under
time.h however musl treat it as a sys timex extension.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210118063808.12471-2-jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

configure

@@ -4039,6 +4039,7 @@ fi
 clock_adjtime=no
 cat > $TMPC <<EOF
 #include <time.h>
+#include <sys/timex.h>
 
 int main(void)
 {