gpiolib: Drop unused forward declaration from driver.h

There is no struct device_node pointers anywhere in the header,
drop unused forward declaration.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

include/linux/gpio/driver.h

@@ -16,7 +16,6 @@
 
 struct gpio_desc;
 struct of_phandle_args;
-struct device_node;
 struct seq_file;
 struct gpio_device;
 struct module;