make sigtimedwait a cancellation point

src/signal/sigtimedwait.c

 #include <signal.h>
 #include <errno.h>
 #include "syscall.h"
+#include "libc.h"
 
 int sigtimedwait(const sigset_t *mask, siginfo_t *si, const struct timespec *timeout)
 {
 	int ret;
+	CANCELPT_BEGIN;
 	do {
 		ret = syscall4(__NR_rt_sigtimedwait, (long)mask, (long)si, (long)timeout, SYSCALL_SIGSET_SIZE);
 	} while (ret<0 && errno==EINTR);
+	CANCELPT_END;
 	return ret;
 }