serial: core: drop unnecessary gpio include

Drop the recently added gpio include from the serial-core header in
favour of a forward declaration and instead include the gpio header only
where needed.

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20200610155121.14014-1-johan@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>