Clear unused variable warning:

external/libese/tools/ese_relay/ese_relay.c:73:7: warning: unused variable 'client_fd' [-Wunused-variable]
  int client_fd = -1;
      ^
1 warning generated.

Test: make checkbuild
Change-Id: Ib40c519bce093c522e7ab648a70a8a9b86d39af2