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: 7ca7a7b9b6 ("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>

arch/riscv/kernel/vector.c

@@ -309,7 +309,7 @@ static int __init riscv_v_sysctl_init(void)
 static int __init riscv_v_sysctl_init(void) { return 0; }
 #endif /* ! CONFIG_SYSCTL */
 
-static int riscv_v_init(void)
+static int __init riscv_v_init(void)
 {
 	return riscv_v_sysctl_init();
 }