powerpc/kvm: Force selection of CONFIG_PPC_FPU

book3s/32 kvm is designed with the assumption that
an FPU is always present.

Force selection of FPU support in the kernel when
build KVM.

Fixes: 7d68c891 ("powerpc/32s: Allow deselecting CONFIG_PPC_FPU on mpc832x")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/74461a99fa1466f361532ca794ca0753be3d9f86.1611038044.git.christophe.leroy@csgroup.eu