x86: stop exporting msr-index.h to userland

Even if this file was not in an uapi directory, it was exported because
it was listed in the Kbuild file.

Fixes: b72e7464

 ("x86/uapi: Do not export <asm/msr-index.h> as part of the user API headers")
Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>