serial: blackfin: Fix missing gpio.h

If CONFIG_SERIAL_BFIN_CTSRTS is set, compile fails because of missing
declarations for the gpio_* api. Include necessary header.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>