ovs_events: fix build failures on Jessie

ovs_events: fix build failures on Jessie

src/utils_ovs.c: In function ‘ovs_db_table_update_cb’:
src/utils_ovs.c:464:15: error: unused variable ‘table_name’ [-Werror=unused-variable]
   const char *table_name = NULL;
               ^~~~~~~~~~
src/utils_ovs.c:463:10: error: unused variable ‘obj_len’ [-Werror=unused-variable]
   size_t obj_len = 0;
          ^~~~~~~
src/utils_ovs.c:462:12: error: unused variable ‘jtable_update’ [-Werror=unused-variable]
   yajl_val jtable_update;
            ^~~~~~~~~~~~~
src/utils_ovs.c: In function ‘ovs_db_reconnect’:
src/utils_ovs.c:713:22: error: unused variable ‘saunix’ [-Werror=unused-variable]
   struct sockaddr_un saunix;
                      ^~~~~~
src/utils_ovs.c:710:14: error: unused variable ‘unix_prefix’ [-Werror=unused-variable]
   const char unix_prefix[] = "unix:";
              ^~~~~~~~~~~
src/utils_ovs.c: In function ‘ovs_yajl_gen_val’:
src/utils_ovs.c:373:10: error: ‘yajl_gen_ret’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   return yajl_gen_ret;
          ^~~~~~~~~~~~
cc1: all warnings being treated as errors