qapi/qnull: Add own header

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20171114180128.17076-2-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
diff --git a/qapi/string-input-visitor.c b/qapi/string-input-visitor.c
index 67a0a4a..b3fdd08 100644
--- a/qapi/string-input-visitor.c
+++ b/qapi/string-input-visitor.c
@@ -16,6 +16,7 @@
 #include "qapi/string-input-visitor.h"
 #include "qapi/visitor-impl.h"
 #include "qapi/qmp/qerror.h"
+#include "qapi/qmp/qnull.h"
 #include "qemu/option.h"
 #include "qemu/queue.h"
 #include "qemu/range.h"