libfdt: Fix C++ compile-time cast error on gnu 4.2.1

Allow the inclusion of libfdt.h in C++ source.

Signed-off-by: Laurent Gregoire <laurent.gregoire@tomtom.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>

include/libfdt.h

@@ -156,7 +156,7 @@ int fdt_next_node(const void *fdt, int offset, int *depth);
 #define __fdt_set_hdr(name) \
 	static inline void fdt_set_##name(void *fdt, uint32_t val) \
 	{ \
-		struct fdt_header *fdth = fdt; \
+		struct fdt_header *fdth = (struct fdt_header*)fdt; \
 		fdth->name = cpu_to_fdt32(val); \
 	}
 __fdt_set_hdr(magic);