KVM: x86: VMX: redo fix for link error without CONFIG_HYPERV

Arnd had sent this patch to the KVM mailing list, but it slipped through
the cracks of maintainers hand-off, and therefore wasn't included in
the pull request.

The same issue had been fixed by Linus in commit dbee3d02

 ("KVM: x86:
VMX: fix build without hyper-v", 2018-06-12) as a self-described
"quick-and-hacky build fix".  However, checking the compile-time
configuration symbol with IS_ENABLED is cleaner and it is enough to
avoid the link error, so switch to Arnd's solution.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[Rewritten commit message. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>