net: fixed_phy: Add forward declaration for struct gpio_desc;

Add forward declaration for struct gpio_desc in order to address
the following:

./include/linux/phy_fixed.h:48:17: error: 'struct gpio_desc' declared inside parameter list [-Werror]
./include/linux/phy_fixed.h:48:17: error: its scope is only this definition or declaration, which is probably not what you want [-Werror]

Fixes: 71bd106d ("net: fixed-phy: Add fixed_phy_register_with_gpiod() API")
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>