Referer module: added $invalid_referer to variables hash.

This makes it accessible via dynamic lookup with ngx_http_get_variable()
from Perl, SSI, etc.

Patch by Yichun Zhang (agentzh).

src/http/modules/ngx_http_referer_module.c

diff --git a/src/http/modules/ngx_http_referer_module.c b/src/http/modules/ngx_http_referer_module.c
index d8a014c..d18b8b9 100644
--- a/src/http/modules/ngx_http_referer_module.c
+++ b/src/http/modules/ngx_http_referer_module.c
@@ -396,8 +396,7 @@
 
     ngx_str_set(&name, "invalid_referer");
 
-    var = ngx_http_add_variable(cf, &name,
-                                NGX_HTTP_VAR_CHANGEABLE|NGX_HTTP_VAR_NOHASH);
+    var = ngx_http_add_variable(cf, &name, NGX_HTTP_VAR_CHANGEABLE);
     if (var == NULL) {
         return NGX_CONF_ERROR;
     }