Fix two warnings when libserialport is not used.

    CC       src/fallback.lo
  src/fallback.c: In function 'sr_serial_list':
  src/fallback.c:26:59: warning: unused parameter 'driver' [-Wunused-parameter]
   SR_API GSList *sr_serial_list(const struct sr_dev_driver *driver)
                                                             ^
  src/fallback.c: In function 'sr_serial_free':
  src/fallback.c:31:51: warning: unused parameter 'serial' [-Wunused-parameter]
   SR_API void sr_serial_free(struct sr_serial_port *serial)