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 <patrick@georgi.software>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58534
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

util/sconfig/sconfig.h

diff --git a/util/sconfig/sconfig.h b/util/sconfig/sconfig.h
index 119d7b5..3ca13b3 100644
--- a/util/sconfig/sconfig.h
+++ b/util/sconfig/sconfig.h
@@ -77,7 +77,7 @@
 	struct reg *reg;
 };
 
-struct device *head;
+extern struct device *head;
 
 struct header;
 struct header {