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>

arch/um/include/shared/os.h

@@ -214,7 +214,6 @@ extern int os_protect_memory(void *addr, unsigned long len,
 extern int os_unmap_memory(void *addr, int len);
 extern int os_drop_memory(void *addr, int length);
 extern int can_drop_memory(void);
-extern void os_flush_stdout(void);
 extern int os_mincore(void *addr, unsigned long len);
 
 /* execvp.c */