rust: kbuild: Add -fdiagnostics-show-context to bindgen_skip_c_flags

rust: kbuild: Add -fdiagnostics-show-context to bindgen_skip_c_flags

This got added with:

  7454048db27d ("kbuild: Enable GCC diagnostic context for value-tracking warnings")

but clang does not have this option, so avoid passing it to bindgen.

[ Details about what the option does are in the commit above. Nathan
  also expands on this:

    Right, this does look correct, as this option is specific to GCC
    for the purpose of exposing more information from GCC internals to
    the user for understanding diagnostics better.

  I checked that in Compiler Explorer GCC 15.2 doesn't have it, but GCC
  trunk indeed has. - Miguel ]

Fixes: 7454048db27d ("kbuild: Enable GCC diagnostic context for value-tracking warnings")
Signed-off-by: Siddhesh Poyarekar <siddhesh@gotplt.org>
Link: https://patch.msgid.link/20251217224050.1186896-1-siddhesh@gotplt.org
[ Removed Cc: stable. Added title prefix. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>