RISC-V: paravirt: steal_time should be static

steal_time is not used outside paravirt.c, make it static,
as sparse suggested.

Fixes: fdf68acc

 ("RISC-V: paravirt: Implement steal-time support")
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>