Include qapi/qmp/qstring.h exactly where needed

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180201111846.21846-14-armbru@redhat.com>
diff --git a/block.c b/block.c
index 367641e..43cc8bc 100644
--- a/block.c
+++ b/block.c
@@ -35,6 +35,7 @@
 #include "qapi/qmp/qerror.h"
 #include "qapi/qmp/qbool.h"
 #include "qapi/qmp/qjson.h"
+#include "qapi/qmp/qstring.h"
 #include "sysemu/block-backend.h"
 #include "sysemu/sysemu.h"
 #include "qemu/notify.h"