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>

include/linux/phy_fixed.h

@@ -11,6 +11,7 @@ struct fixed_phy_status {
 };
 
 struct device_node;
+struct gpio_desc;
 
 #if IS_ENABLED(CONFIG_FIXED_PHY)
 extern int fixed_phy_change_carrier(struct net_device *dev, bool new_carrier);