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

---
 arch/score/include/asm/Kbuild | 1 +
 1 file changed, 1 insertion(+)

index bfb31569..108a225 100644
@@ -10,5 +10,6 @@ generic-y += mcs_spinlock.h
 generic-y += pci.h
 generic-y += preempt.h
 generic-y += scatterlist.h
+generic-y += serial.h
 generic-y += trace_clock.h
 generic-y += xor.h

1.9.2.459.g68773ac