KVM: PPC: fix build warning in kvm_arch_vcpu_ioctl_run

Fix compile warning:
  CC [M]  arch/powerpc/kvm/powerpc.o
  arch/powerpc/kvm/powerpc.c: In function 'kvm_arch_vcpu_ioctl_run':
  arch/powerpc/kvm/powerpc.c:290: warning: 'gpr' may be used uninitialized in this function
  arch/powerpc/kvm/powerpc.c:290: note: 'gpr' was declared here

Signed-off-by: Denis Kirjanov <dkirjanov@kernel.org>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>