RISC-V: Mark riscv_v_init() as __init

RISC-V: Mark riscv_v_init() as __init

[ Upstream commit 9d87cf525fd2e1a5fcbbb40ee3df216d1d266c88 ]

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

Fixes: 7ca7a7b9b635 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>