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>