x86: Unconfuse CONFIG_ and X86_FEATURE_ namespaces

Lukas reported someone fat fingered the CONFIG_ symbol; fix er up.

Fixes: 5d821386 ("x86/retbleed: Add SKL return thunk")
Reported-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Y1+fL4qQEIGZEEKB@hirez.programming.kicks-ass.net