um: Remove the repeated declaration

Function 'os_flush_stdout' is declared twice, so remove the
repeated declaration.

Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Richard Weinberger <richard@nod.at>