libfdt: Add missing functions to shared library

The commit 4e76ec7 "libfdt: Add fdt_next_subnode() to permit easy
subnode iteration" adds new functions (fdt_{first,next}_subnode) but
forgot to mark them as 'global' in the shared library.

Signed-off-by: Julien Grall <julien.grall@linaro.org>

libfdt/version.lds

@@ -54,6 +54,8 @@ LIBFDT_1.2 {
 		fdt_get_property_by_offset;
 		fdt_getprop_by_offset;
 		fdt_next_property_offset;
+		fdt_first_subnode;
+		fdt_next_subnode;
 
 	local:
 		*;