fix building --without-http-cache, broken in r2930

src/http/modules/ngx_http_proxy_module.c

diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index f0aadb4..f2d939e 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -2343,7 +2343,10 @@
     }
 
     if (conf->headers_set_hash.buckets
-        && ((conf->upstream.cache == NULL) == (prev->upstream.cache == NULL)))
+#if (NGX_HTTP_CACHE)
+        && ((conf->upstream.cache == NULL) == (prev->upstream.cache == NULL))
+#endif
+       )
     {
         return NGX_OK;
     }