docs: driver-api: gpio: consumer: Mark another line of code as such

Make it so that this #include line is rendered in monospace, like other
code blocks.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>