h8300: include: asm: Kbuild: add default "serial.h"

Normally, an architecture need support serial, so include the default one, or can not pass compiling:

The related error (allmodconfig for h8300):

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

---
 arch/h8300/include/asm/Kbuild |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

index 8ada3cf..df289fb 100644
@@ -4,5 +4,6 @@ generic-y += exec.h
 generic-y += linkage.h
 generic-y += mmu.h
 generic-y += module.h
+generic-y += serial.h
 generic-y += trace_clock.h
 generic-y += xor.h

1.7.7.6