"KVM: x86: enable -Werror" breaks W=1 compilation

[Qian Cai]

People have been running W=1 to catch additional compilation warnings with an
expectation that not all of warnings will be fixed, but the linux-next commit
ead68df94d24 ("KVM: x86: enable -Werror") breaks the build for them.

arch/x86/kvm/../../../virt/kvm/kvm_main.c:663:12: error: no previous prototype
for âkvm_arch_post_init_vmâ [-Werror=missing-prototypes]
Âint __weak kvm_arch_post_init_vm(struct kvm *kvm)
ÂÂÂÂÂÂÂÂÂÂÂÂ^~~~~~~~~~~~~~~~~~~~~
arch/x86/kvm/../../../virt/kvm/kvm_main.c:672:13: error: no previous prototype
for âkvm_arch_pre_destroy_vmâ [-Werror=missing-prototypes]
Âvoid __weak kvm_arch_pre_destroy_vm(struct kvm *kvm)