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/[e-mail]
Reviewed-by: Nicolas Schier <[e-mail]>
Tested-by: Nicolas Schier <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>

include/linux/compiler-version.h

···
 #ifdef GCC_PLUGINS
 #include <generated/gcc-plugins.h>
 #endif
+
+/*
+ * If the randstruct seed itself changes (whether for GCC plugins or
+ * Clang), the entire tree needs to be rebuilt since the randomization of
+ * structures may change between compilation units if not.
+ */
+#ifdef RANDSTRUCT
+#include <generated/randstruct_hash.h>
+#endif

include/linux/vermagic.h

···
 #define MODULE_VERMAGIC_MODVERSIONS ""
 #endif
 #ifdef RANDSTRUCT
-#include <generated/randstruct_hash.h>
 #define MODULE_RANDSTRUCT "RANDSTRUCT_" RANDSTRUCT_HASHED_SEED
 #else
 #define MODULE_RANDSTRUCT