randstruct: Force full rebuild when seed changes

randstruct: Force full rebuild when seed changes

While the randstruct GCC plugin was being rebuilt if the randstruct seed
changed, Clang builds did not notice the change. This could result in
differing struct layouts in a target depending on when it was built.

Include the existing generated header file in compiler-version.h when
its associated feature name, RANDSTRUCT, is defined. This will be picked
up by fixdep and force rebuilds where needed.

Link: https://lore.kernel.org/r/20250503184623.2572355-2-kees@kernel.org
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Tested-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Kees Cook <kees@kernel.org>