superio/serverengines/pilot: Avoid .c includes

Following the same reasoning as commit
d3043313a91dff3bc2f879ffb3b4bf23a364d711 superio/fintek/f81865f: Avoid
.c includes
Clean up the early_serial #include directives in mainboard/romstage code.

Change-Id: Ia6ed36c8517a95b651fefdd855eec0ec91d73187
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5439
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>