Monitor: Convert do_screen_dump() to QObject

Trivial, as it never fails, doesn't have output nor return
any data.

Note that it's also available under QMP, as kvm-autotest
needs this.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/monitor.c b/monitor.c
index 072bea4..ba60f97 100644
--- a/monitor.c
+++ b/monitor.c
@@ -1164,9 +1164,10 @@
     return ret;
 }
 
-static void do_screen_dump(Monitor *mon, const QDict *qdict)
+static int do_screen_dump(Monitor *mon, const QDict *qdict, QObject **ret_data)
 {
     vga_hw_screen_dump(qdict_get_str(qdict, "filename"));
+    return 0;
 }
 
 static void do_logfile(Monitor *mon, const QDict *qdict)