RISC-V: KVM: add __init annotation to riscv_kvm_init()

The riscv_kvm_init() is a module_init entry so let us add __init
annotation to it.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Anup Patel <anup@brainfault.org>