fix coreboot/fam15h build error on arch

In file included from /home/user/lbmk/src/coreboot/fam15h/util/cbfstool/partitioned_file.h:19,
                 from /home/user/lbmk/src/coreboot/fam15h/util/cbfstool/partitioned_file.c:16:
/home/user/lbmk/src/coreboot/fam15h/util/cbfstool/common.h:34:16: error: expected ‘)’ before ‘__attribute__’
   34 | #define unused __attribute__((unused))
      |                ^~~~~~~~~~~~~
In file included from /home/user/lbmk/src/coreboot/fam15h/util/cbfstool/common.h:25:
/home/user/lbmk/src/coreboot/fam15h/src/commonlib/include/commonlib/helpers.h:137:40: error: expected identifier or ‘(’ before ‘)’ token
  137 | #define __unused __attribute__((unused))

^ this removes that error

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/coreboot/fam15h/patches/0018-fix-arch-linux-build-error.patch

@@ -0,0 +1,30 @@
+From 0e6a5b0e82309715683ce3033dc34897852827e5 Mon Sep 17 00:00:00 2001
+From: Leah Rowe <leah@libreboot.org>
+Date: Wed, 1 Apr 2026 08:25:07 +0100
+Subject: [PATCH 1/1] fix arch linux build error
+
+fixes cbfstool builds on arch linux on this old coreboot
+
+Signed-off-by: Leah Rowe <leah@libreboot.org>
+---
+ src/commonlib/include/commonlib/helpers.h | 4 ----
+ 1 file changed, 4 deletions(-)
+
+diff --git a/src/commonlib/include/commonlib/helpers.h b/src/commonlib/include/commonlib/helpers.h
+index ca3b3c58f9..e28b5c4c37 100644
+--- a/src/commonlib/include/commonlib/helpers.h
++++ b/src/commonlib/include/commonlib/helpers.h
+@@ -133,10 +133,6 @@
+ /* Calculate size of structure member. */
+ #define member_size(type, member)	(sizeof(((type *)0)->member))
+ 
+-#ifndef __unused
+-#define __unused __attribute__((unused))
+-#endif
+-
+ #ifndef alloca
+ #define alloca(x) __builtin_alloca(x)
+ #endif
+-- 
+2.47.3
+