monitor/hmp: Explicit we ignore a QEMUChrEvent in IOEventHandler

The Chardev events are listed in the QEMUChrEvent enum. To be
able to use this enum in the IOEventHandler typedef, we need to
explicit an event ignored by this frontend, to silent the
following GCC warning:

    CC      monitor/hmp.o
  monitor/hmp.c: In function ‘monitor_event’:
  monitor/hmp.c:1330:5: error: enumeration value ‘CHR_EVENT_BREAK’ not handled in switch [-Werror=switch]
   1330 |     switch (event) {
        |     ^~~~~~
  cc1: all warnings being treated as errors

Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20191218172009.8868-13-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

monitor/hmp.c

@@ -1371,6 +1371,10 @@ static void monitor_event(void *opaque, int event)
         mon_refcount--;
         monitor_fdsets_cleanup();
         break;
+
+    case CHR_EVENT_BREAK:
+        /* Ignored */
+        break;
     }
 }