MonitorFdHup::~MonitorFdHup: use proper close method instead of libc close()

Otherwise closing it again will cause an EBADF in the AutoCloseFd class.

(cherry picked from commit 87a34a45ff)

src/libutil/unix/monitor-fd.hh

@@ -122,7 +122,7 @@ public:
 
     ~MonitorFdHup()
     {
-        close(notifyPipe.writeSide.get());
+        notifyPipe.writeSide.close();
         thread.join();
     }
 };