migration: Fix warning caused by missing declaration of vmstate_dummy

Warning from the Sparse static analysis tool:

stubs/vmstate.c:4:26: warning:
 symbol 'vmstate_dummy' was not declared. Should it be static?

Cc: Juan Quintela <quintela@redhat.com>
Cc: Amit Shah <amit.shah@redhat.com>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

include/migration/vmstate.h

@@ -139,9 +139,7 @@ struct VMStateDescription {
     const VMStateSubsection *subsections;
 };
 
-#ifdef CONFIG_USER_ONLY
 extern const VMStateDescription vmstate_dummy;
-#endif
 
 extern const VMStateInfo vmstate_info_bool;