RISC-V: Mark riscv_v_init() as __init

This trips up with Xtheadvector enabled, but as far as I can tell it's
just been an issue since the original patchset.

Fixes: 7ca7a7b9 ("riscv: Add sysctl to set the default vector rule for new processes")
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20250115180251.31444-1-palmer@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>