kbuild: rebuild GCC plugins when the compiler is upgraded

Linus reported a build error due to the GCC plugin incompatibility
when the compiler is upgraded. [1]

GCC plugins are tied to a particular GCC version. So, they must be
rebuilt when the compiler is upgraded.

This seems to be a long-standing flaw since the initial support of
GCC plugins.

Extend commit 8b59cd81dc ("kbuild: ensure full rebuild when the
compiler is updated"), so that GCC plugins are covered by the
compiler upgrade detection.

[1]: https://lore.kernel.org/lkml/CAHk-=wieoN5ttOy7SnsGwZv+Fni3R6m-Ut=oxih6bbZ28G+4dw@mail.gmail.com/

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>

Makefile

@@ -479,6 +479,7 @@ USERINCLUDE    := \
 		-I$(objtree)/arch/$(SRCARCH)/include/generated/uapi \
 		-I$(srctree)/include/uapi \
 		-I$(objtree)/include/generated/uapi \
+                -include $(srctree)/include/linux/compiler-version.h \
                 -include $(srctree)/include/linux/kconfig.h
 
 # Use LINUXINCLUDE when you must reference the include/ directory.