io/task: Move 'qom/object.h' header to source

We need "qom/object.h" to call object_ref()/object_unref(),
and to test the TYPE_DUMMY.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200504115656.6045-3-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/io/task.h

@@ -21,8 +21,6 @@
 #ifndef QIO_TASK_H
 #define QIO_TASK_H
 
-#include "qom/object.h"
-
 typedef struct QIOTask QIOTask;
 
 typedef void (*QIOTaskFunc)(QIOTask *task,