kbuild: Enable -Wuninitialized

This helps fine very dodgy behavior through both -Wuninitialized
(warning that a variable is always uninitialized) and
-Wsometimes-uninitialized (warning that a variable is sometimes
uninitialized, like GCC's -Wmaybe-uninitialized). These warnings
catch things that GCC doesn't such as:

https://lore.kernel.org/lkml/86649ee4-9794-77a3-502c-f4cd10019c36@lca.pw/

We very much want to catch these so turn this warning on so that CI is
aware of it.

Link: https://github.com/ClangBuiltLinux/linux/issues/381
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>