buildsystem: Promote rules.h to default include

Does not fix 3rdparty/, *.S or *.ld or yet.

Change-Id: I66b48013dd89540b35ab219d2b64bc13f5f19cda
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/17656
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/lib/timestamp.c

@@ -23,7 +23,6 @@
 #include <timer.h>
 #include <timestamp.h>
 #include <arch/early_variables.h>
-#include <rules.h>
 #include <smp/node.h>
 
 #define MAX_TIMESTAMPS 192