of/flattree: forward declare struct device_node in of_fdt.h

This patch forward declares struct device_node to fix a compile error
when of_fdt.h is included, but of.h is not.  Alternately, including
linux/of.h could have been added to of_fdt.h, but that pulls in a lot
of unnecessary declarations when only working with the flattened
form.
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

include/linux/of_fdt.h

@@ -59,6 +59,8 @@ struct boot_param_header {
 
 #if defined(CONFIG_OF_FLATTREE)
 
+struct device_node;
+
 /* For scanning an arbitrary device-tree at any time */
 extern char *of_fdt_get_string(struct boot_param_header *blob, u32 offset);
 extern void *of_fdt_get_property(struct boot_param_header *blob,