superio/fintek/f81865f: Avoid .c includes

We should not be #include .c files, instead link early_serial into
romstage and provide a prototype.

Change-Id: Ia9277169ce1592e1fc72f8849f0982741daec567
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5416
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)