KVM: PPC: Fix compilation when KVM is not enabled

Compiling with CONFIG_PPC_POWERNV=y and KVM disabled currently gives
an error like this:

  CC      arch/powerpc/kernel/dbell.o
In file included from arch/powerpc/kernel/dbell.c:20:0:
arch/powerpc/include/asm/kvm_ppc.h: In function ‘xics_on_xive’:
arch/powerpc/include/asm/kvm_ppc.h:625:9: error: implicit declaration of function ‘xive_enabled’ [-Werror=implicit-function-declaration]
  return xive_enabled() && cpu_has_feature(CPU_FTR_HVMODE);
         ^
cc1: all warnings being treated as errors
scripts/Makefile.build:276: recipe for target 'arch/powerpc/kernel/dbell.o' failed
make[3]: *** [arch/powerpc/kernel/dbell.o] Error 1

Fix this by making the xics_on_xive() definition conditional on the
same symbol (CONFIG_KVM_BOOK3S_64_HANDLER) that determines whether we
include <asm/xive.h> or not, since that's the header that defines
xive_enabled().

Fixes: 03f95332 ("KVM: PPC: Book3S: Allow XICS emulation to work in nested hosts using XIVE")
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/include/asm/kvm_ppc.h

@@ -619,7 +619,7 @@ static inline int kvmppc_xive_set_irq(struct kvm *kvm, int irq_source_id, u32 ir
 static inline void kvmppc_xive_push_vcpu(struct kvm_vcpu *vcpu) { }
 #endif /* CONFIG_KVM_XIVE */
 
-#ifdef CONFIG_PPC_POWERNV
+#if defined(CONFIG_PPC_POWERNV) && defined(CONFIG_KVM_BOOK3S_64_HANDLER)
 static inline bool xics_on_xive(void)
 {
 	return xive_enabled() && cpu_has_feature(CPU_FTR_HVMODE);