arch/score/include/asm/Kbuild: Add generic "serial.h"

The related error (with allmodconfig under score):

    CC [M]  drivers/staging/speakup/speakup_acntpc.o
  In file included from drivers/staging/speakup/speakup_acntpc.c:33:0:
  drivers/staging/speakup/serialio.h:7:24: fatal error: asm/serial.h: No such file or directory
   #include <asm/serial.h>
                          ^
  compilation terminated.
  make[3]: *** [drivers/staging/speakup/speakup_acntpc.o] Error 1
  make[2]: *** [drivers/staging/speakup] Error 2
  make[1]: *** [drivers/staging] Error 2
  make: *** [drivers] Error 2

Acked-by: Lennox Wu <lennox.wu@gmail.com>
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>