kbuild: enable -Wcast-function-type-strict unconditionally

All known function cast warnings are now addressed, so the warning can be
enabled globally to catch new ones more quickly.

Link: https://lkml.kernel.org/r/20240415122037.1983124-6-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>