Otherwise we have build errors.

   wrc_main.c:172: undefined reference to `uart_init_hw'
   wrc_main.c:172: undefined reference to `uart_read_byte'
   ppsi/arch-wrpc/wrc_ptp_ppsi.c:103: undefined reference to `uart_init_hw'

If we want to make it optional, we can revert this commit
(thanks randconfig)
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>