Fix a build warning with stasis messages.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@392032 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/app.h

@@ -29,6 +29,7 @@
 #include "asterisk/file.h"
 #include "asterisk/linkedlists.h"
 #include "asterisk/utils.h"
+#include "asterisk/stasis.h"
 
 struct ast_flags64;