libfdt: De-inline fdt_header_size()

There does not seem to be a strong reason to inline this function. Also
we are about to add some extra code to it which will increase its size.

Move it into fdt.c and use a simple declaration in libfdt.h

Signed-off-by: Simon Glass <sjg@chromium.org>
Message-Id: <20200220214557.176528-2-sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>