[clang] [X86] Update #error to mention the right header name (PR #207507)

[Kim Gräsman via cfe-commits]

kimgr wrote:

We're using these `#error` messages heuristically to build IWYU mappings for the intrinsics headers, and one of our contributors noticed this was the only inconsistent one: https://github.com/include-what-you-use/include-what-you-use/pull/2066.

https://github.com/llvm/llvm-project/pull/207507