gpio: stubs in headers should be inline

Fixes: fdd61a01 ("gpio: Add support for hierarchical IRQ domains")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/20190816213812.40a130db@canb.auug.org.au


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>