vnc: add qapi/error.h include to stubs

Fixes --disable-vnc build failure.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20180213070526.22475-1-kraxel@redhat.com

ui/vnc-stubs.c

@@ -1,5 +1,6 @@
 #include "qemu/osdep.h"
 #include "ui/console.h"
+#include "qapi/error.h"
 
 int vnc_display_password(const char *id, const char *password)
 {