in_nginx_exporter_metrics: fixed a wrong data type used for an option

Signed-off-by: Leonardo Alminana <leonardo.alminana@chronosphere.io>

plugins/in_nginx_exporter_metrics/nginx.h

@@ -45,7 +45,7 @@ struct nginx_ctx
     struct cmt_gauge *connections_waiting;
     struct cmt_gauge *connection_up;
     bool is_up;
-    bool is_nginx_plus;
+    int is_nginx_plus;
     int nginx_plus_version;
 
     struct nginx_plus_connections *plus_connections;