x86/kvm: fix a missing-prototypes "vmread_error"

The commit 842f4be9 ("KVM: VMX: Add a trampoline to fix VMREAD error
handling") removed the declaration of vmread_error() causes a W=1 build
failure with KVM_WERROR=y. Fix it by adding it back.

arch/x86/kvm/vmx/vmx.c:359:17: error: no previous prototype for 'vmread_error' [-Werror=missing-prototypes]
 asmlinkage void vmread_error(unsigned long field, bool fault)
                 ^~~~~~~~~~~~
Signed-off-by: Qian Cai <cai@lca.pw>
Message-Id: <20200402153955.1695-1-cai@lca.pw>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/ops.h

@@ -12,6 +12,7 @@
 
 #define __ex(x) __kvm_handle_fault_on_reboot(x)
 
+asmlinkage void vmread_error(unsigned long field, bool fault);
 __attribute__((regparm(0))) void vmread_error_trampoline(unsigned long field,
 							 bool fault);
 void vmwrite_error(unsigned long field, unsigned long value);