BUILD: ring: forward-declare struct appctx to avoid a build warning
When using ring.h standalone it emits warnings about appctx. Let's
forward-declare it.
diff --git a/include/haproxy/ring.h b/include/haproxy/ring.h
index a3a55d8..9e87407 100644
--- a/include/haproxy/ring.h
+++ b/include/haproxy/ring.h
@@ -26,6 +26,8 @@
#include <import/ist.h>
#include <haproxy/ring-t.h>
+struct appctx;
+
struct ring *ring_new(size_t size);
void ring_init(struct ring *ring, void* area, size_t size);
struct ring *ring_resize(struct ring *ring, size_t size);