audit: printf warning fix

fix warning
  CC     libvirt_util_la-virtaudit.lo
cc1: warnings being treated as errors
util/virtaudit.c: In function 'virAuditEncode':
util/virtaudit.c:146: error: implicit declaration of function 'virAsprintf' [-Wimplicit-function-declaration]
util/virtaudit.c:146: error: nested extern declaration of 'virAsprintf' [-Wnested-externs]

AUTHORS

@@ -134,6 +134,7 @@ Patches have also been contributed by:
   Harsh Prateek Bora   <harsh@linux.vnet.ibm.com>
   John Morrissey       <jwm@horde.net>
   Osier Yang           <jyang@redhat.com>
+  Kamezawa Hiroyuki    <kamezawa.hiroyu@jp.fujitsu.com>
 
   [....send patches to get your name here....]
 

src/util/virtaudit.c

@@ -30,6 +30,7 @@
 #include "virterror_internal.h"
 #include "logging.h"
 #include "virtaudit.h"
+#include "util.h"
 
 /* Provide the macros in case the header file is old.
    FIXME: should be removed. */