KVM: PPC: do not compile in vfio.o unconditionally

Build on 32-bit PPC fails with the following error:

int kvm_vfio_ops_init(void)
^
In file included from arch/powerpc/kvm/../../../virt/kvm/vfio.c:21:0:
arch/powerpc/kvm/../../../virt/kvm/vfio.h:8:90: note: previous definition of ‘kvm_vfio_ops_init’ was here
arch/powerpc/kvm/../../../virt/kvm/vfio.c:292:6: error: redefinition of ‘kvm_vfio_ops_exit’
void kvm_vfio_ops_exit(void)
^
In file included from arch/powerpc/kvm/../../../virt/kvm/vfio.c:21:0:
arch/powerpc/kvm/../../../virt/kvm/vfio.h:12:91: note: previous definition of ‘kvm_vfio_ops_exit’ was here
scripts/Makefile.build:258: recipe for target arch/powerpc/kvm/../../../virt/kvm/vfio.o failed
make[3]: *** [arch/powerpc/kvm/../../../virt/kvm/vfio.o] Error 1

Check whether CONFIG_KVM_VFIO is set before including vfio.o
in the build.

Reported-by: Pranith Kumar <[e-mail]>
Tested-by: Pranith Kumar <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/powerpc/kvm/Makefile

···
 KVM := ../../../virt/kvm
 
 common-objs-y = $(KVM)/kvm_main.o $(KVM)/coalesced_mmio.o \
-		$(KVM)/eventfd.o $(KVM)/vfio.o
+		$(KVM)/eventfd.o
+common-objs-$(CONFIG_KVM_VFIO) += $(KVM)/vfio.o
 
 CFLAGS_e500_mmu.o := -I.
 CFLAGS_e500_mmu_host.o := -I.