Request body: unbreak build without debug.

src/http/ngx_http_request_body.c

diff --git a/src/http/ngx_http_request_body.c b/src/http/ngx_http_request_body.c
index cb3f598..931694e 100644
--- a/src/http/ngx_http_request_body.c
+++ b/src/http/ngx_http_request_body.c
@@ -976,7 +976,9 @@
 static ngx_int_t
 ngx_http_request_body_save_filter(ngx_http_request_t *r, ngx_chain_t *in)
 {
+#if (NGX_DEBUG)
     ngx_chain_t               *cl;
+#endif
     ngx_http_request_body_t   *rb;
 
     rb = r->request_body;