libfdt: fdt_create_with_flags(): Fix comparison warning

With -Wsign-compare, compilers warn about a mismatching signedness
in a comparison in fdt_create_with_flags().

By making hdrsize a signed integer (we are sure it's a very small
number), we avoid all the casts and have matching types.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Message-Id: <20201001164630.4980-4-andre.przywara@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>