auxdisplay: make charlcd_backlight visible to hd44780_common

hd44780_common wants to use the charlcd_backlight function, so make it
visible.

Reviewed-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>