MINOR: cache: Always access the stream-int via the conn-stream

To be able to move the stream-interface from the stream to the conn-stream,
all access to the SI is done via the conn-stream. This patch is limited to
the cache part.
diff --git a/src/cache.c b/src/cache.c
index b4918e7..05f81e5 100644
--- a/src/cache.c
+++ b/src/cache.c
@@ -1812,7 +1812,7 @@
 		}
 
 		s->target = &http_cache_applet.obj_type;
-		if ((appctx = si_register_handler(&s->si[1], objt_applet(s->target)))) {
+		if ((appctx = si_register_handler(cs_si(s->csb), objt_applet(s->target)))) {
 			appctx->st0 = HTX_CACHE_INIT;
 			appctx->rule = rule;
 			appctx->ctx.cache.entry = res;