stubs: Fix warning caused by missing include statement

Warning from the Sparse static analysis tool:

stubs/qtest.c:14:6:
 warning: symbol 'qtest_allowed' was not declared. Should it be static?

Add the missing include statement which declares qtest_allowed.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

stubs/qtest.c

@@ -8,7 +8,7 @@
  * See the COPYING file in the top-level directory.
  */
 
-#include "qemu-common.h"
+#include "sysemu/qtest.h"
 
 /* Needed for qtest_allowed() */
 bool qtest_allowed;