drivers/elog: Fix implicit function declaration issue

Fix compilation. Relying on the pre-processor to condition an if
statement will lead to warnings of implicitly defined functions. To
solve this dilemma add symbols to resolve to at compile time.

Change-Id: Id0117528c5579cc1dec750a8a17a76fab4314b3f
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/5504
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>