KVM: x86: enable -Werror

Avoid more embarrassing mistakes.  At least those that the compiler
can catch.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
 ccflags-y += -Iarch/x86/kvm
+ccflags-y += -Werror
 
 KVM := ../../../virt/kvm