linux/kernel/common-config.nix: mark FORTIFY_SOURCE as optional
linux/kernel/common-config.nix: mark FORTIFY_SOURCE as optional

You cannot use it on clang-built kernels due to some LLVM bugs, namely:

* https://bugs.llvm.org/show_bug.cgi?id=50322
* https://bugs.llvm.org/show_bug.cgi?id=41459

so Kconfig forces it off, causing generate-config.pl to explode since it
is not marked optional.

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