gpiolib: devprop: Fix syntax error

EXPORT_SYMBOL_GPL() needs a semicolon at the end, also
explicitly include <linux/export.h> as well.

Fixes: 64ebde5b ("gpiolib: export devprop_gpiochip_set_names()")
Acked-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>