x86/defconfig: Enable WERROR

To quote Linus:

  "EVERYBODY should have CONFIG_WERROR=y on at least x86-64 and other
   serious architectures, unless you have some completely random
   experimental (and broken) compiler.

New compiler warnings are not acceptable."

So this should make at least the most obvious and common ones not go
unnoticed.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/YjsCpoRK7W4l6tSh@zn.tnic