KVM: MIPS: Fix a build error for !CPU_LOONGSON64

During the KVM merging progress, a CONFIG_CPU_LOONGSON64 guard in commit
7f2a83f1 ("KVM: MIPS: Add CPUCFG emulation for Loongson-3")
is missing by accident. So add it to avoid building error.

Fixes: 7f2a83f1

 ("KVM: MIPS: Add CPUCFG emulation for Loongson-3")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Message-Id: <1592204215-28704-1-git-send-email-chenhc@lemote.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>