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>
1 file changed