missing-syscall: fix copypasta

As noticed by @mbiebl:

5134e18eed (r43033443)

src/basic/missing_syscall.h

@@ -660,7 +660,7 @@ assert_cc(__NR_pidfd_send_signal == systemd_NR_pidfd_send_signal);
 
 #if !HAVE_PIDFD_SEND_SIGNAL
 static inline int missing_pidfd_send_signal(int fd, int sig, siginfo_t *info, unsigned flags) {
-#  ifdef __NR_pidfd_open
+#  ifdef __NR_pidfd_send_signal
         return syscall(__NR_pidfd_send_signal, fd, sig, info, flags);
 #  else
         errno = ENOSYS;