mainboard/intel/cougar_canyon2/romstage.c: Missing prototype

Fix warning thrown by Clang due to missing prototype for main
entry point function in -ffreestanding. main() is as any other
function in freestanding and so a prototype is strictly needed.

Change-Id: Icb29ced0306d5089049a35b1d8862f86a555ff1f
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7517
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>