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

It's needed for proxies_list and used to be inherited via cfgparse.h.
diff --git a/src/filters.c b/src/filters.c
index 2075bee..b684b44 100644
--- a/src/filters.c
+++ b/src/filters.c
@@ -21,6 +21,7 @@
 #include <haproxy/http_htx.h>
 #include <haproxy/htx.h>
 #include <haproxy/namespace.h>
+#include <haproxy/proxy.h>
 #include <haproxy/stream.h>
 #include <haproxy/stream_interface.h>
 #include <haproxy/tools.h>