unneeded internal declaration

[Nick Desaulniers]

today I noticed I was getting the warning:

arch/x86/kvm/vmx.c:64:32: warning: variable 'vmx_cpu_id' is not needed
and will not be emitted [-Wunneeded-internal-declaration]

seems like this was added in commit: e9bda3b3d0ce7 "KVM: VMX:
Auto-load on CPUs with VMX"

seems like other call sites of the MODULE_DEVICE_TABLE typically get
added to an id_table of various driver structs.  Should that be the
case here, or would a `__unused` modifier be a way forward (if so,
please confirm, would be a good first bug for a friend)?