util/sconfig: Avoid duplicate symbols in link step Recent linkers became more picky about unifying identical symbols, so avoid struct *device head to appear several times by marking it "extern". Follows a change in commit 79e8412665567c, but without carrying all the other refactorings done there. Change-Id: Icede0936cc99f1c183c09812bffc3cc9a3993efa Signed-off-by: Patrick Georgi <[email protected]> Reviewed-on: https://review.coreboot.org/c/coreboot/+/58534 Tested-by: build bot (Jenkins) <[email protected]> Reviewed-by: Martin Roth <[email protected]>