qapi: add missing trace_visit_type_enum() call

A trace event exists for enums but it's never called.  This patch fixes
this oversight so that enums are traced just like the other QAPI types.

Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20170126171613.1399-2-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

qapi/qapi-visit-core.c

@@ -374,6 +374,7 @@ void visit_type_enum(Visitor *v, const char *name, int *obj,
                      const char *const strings[], Error **errp)
 {
     assert(obj && strings);
+    trace_visit_type_enum(v, name, obj);
     switch (v->type) {
     case VISITOR_INPUT:
         input_type_enum(v, name, obj, strings, errp);