x86: Export asm/{svm.h,vmx.h,perf_regs.h}

Export asm/{svm.h,vmx.h,perf_regs.h} so that they can be disintegrated.

It looks from previous commits that the first two should have been exported,
but the header-y lines weren't added to the Kbuild.

I'm guessing that asm/perf_regs.h should be exported too.

Signed-off-by: David Howells <dhowells@redhat.com>