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]