Core: added a stub for additional zone configuration.

src/core/ngx_cycle.h

diff --git a/src/core/ngx_cycle.h b/src/core/ngx_cycle.h
index a825da2..54fa2e6 100644
--- a/src/core/ngx_cycle.h
+++ b/src/core/ngx_cycle.h
@@ -31,6 +31,7 @@
     ngx_shm_t                 shm;
     ngx_shm_zone_init_pt      init;
     void                     *tag;
+    void                     *sync;
     ngx_uint_t                noreuse;  /* unsigned  noreuse:1; */
 };