arch: hexagon: include: asm: Kbuild: add generic "serial.h" in Kbuild

Add "serial.h" in Kbuild, or can not pass compiling with allmodconfig,
the related error:

    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
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>

arch/hexagon/include/asm/Kbuild

@@ -39,6 +39,7 @@ generic-y += scatterlist.h
 generic-y += sections.h
 generic-y += segment.h
 generic-y += sembuf.h
+generic-y += serial.h
 generic-y += shmbuf.h
 generic-y += shmparam.h
 generic-y += siginfo.h