BUILD: sink: include proxy.h in sink.c

It's needed for proxies_list but was missing.
diff --git a/src/sink.c b/src/sink.c
index 78091aa..6358da5 100644
--- a/src/sink.c
+++ b/src/sink.c
@@ -25,6 +25,7 @@
 #include <haproxy/errors.h>
 #include <haproxy/list.h>
 #include <haproxy/log.h>
+#include <haproxy/proxy.h>
 #include <haproxy/ring.h>
 #include <haproxy/signal.h>
 #include <haproxy/sink.h>