Include serial.h in ser-base.h.

Include serial.h in ser-base.h.

$ make check-headers CHECK_HEADERS="ser-base.h"
...
../../binutils-gdb/gdb/ser-base.h:33:8: error: 'serial_ttystate' does not name a type
 extern serial_ttystate ser_base_get_tty_state (struct serial *scb);
        ^

gdb:

2017-01-06  Yao Qi  <yao.qi@linaro.org>

	* ser-base.h: Include serial.h.