KVM: selftests: Fix build for evmcs.h

I got this error when building kvm selftests:

/usr/bin/ld: /home/xz/git/linux/tools/testing/selftests/kvm/libkvm.a(vmx.o):/home/xz/git/linux/tools/testing/selftests/kvm/include/evmcs.h:222: multiple definition of `current_evmcs'; /tmp/cco1G48P.o:/home/xz/git/linux/tools/testing/selftests/kvm/include/evmcs.h:222: first defined here
/usr/bin/ld: /home/xz/git/linux/tools/testing/selftests/kvm/libkvm.a(vmx.o):/home/xz/git/linux/tools/testing/selftests/kvm/include/evmcs.h:223: multiple definition of `current_vp_assist'; /tmp/cco1G48P.o:/home/xz/git/linux/tools/testing/selftests/kvm/include/evmcs.h:223: first defined here

I think it's because evmcs.h is included both in a test file and a lib file so
the structs have multiple declarations when linking. After all it's not a good
habit to declare structs in the header files.

Cc: Vitaly Kuznetsov <[e-mail]>
Signed-off-by: Peter Xu <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

tools/testing/selftests/kvm/include/evmcs.h

···
 #define HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_MASK	\
 		(~((1ull << HV_X64_MSR_VP_ASSIST_PAGE_ADDRESS_SHIFT) - 1))
 
-struct hv_enlightened_vmcs *current_evmcs;
-struct hv_vp_assist_page *current_vp_assist;
+extern struct hv_enlightened_vmcs *current_evmcs;
+extern struct hv_vp_assist_page *current_vp_assist;
 
 int vcpu_enable_evmcs(struct kvm_vm *vm, int vcpu_id);
 

tools/testing/selftests/kvm/lib/x86_64/vmx.c

···
 
 bool enable_evmcs;
 
+struct hv_enlightened_vmcs *current_evmcs;
+struct hv_vp_assist_page *current_vp_assist;
+
 struct eptPageTableEntry {
 	uint64_t readable:1;
 	uint64_t writable:1;