Fix compilation with ENABLE_BACKTRACE=OFF

  src/lua/init.c: In function ‘tarantool_panic_handler’:
  src/lua/init.c:321:2: error: implicit declaration of function ‘print_backtrace’ [-Werror=implicit-function-declaration]
    print_backtrace();
    ^~~~~~~~~~~~~~~

  src/lua/fiber.c:244:1: error: ‘lbox_fiber_statof_bt’ defined but not used [-Werror=unused-function]
   lbox_fiber_statof_bt(struct fiber *f, void *cb_ctx)
   ^~~~~~~~~~~~~~~~~~~~